Validate field and do not close the form

I am working on a plugin for Qgis, in the form the user enters the data and one of them is the name of the file, so I validate the field and verify that the same name does not exist, the problem is that if there is a sample a message and the form is closed, not allowing the user to modify the name.

The function that verifies the name of the document is the following:

def verificaArchivo(self,file):

   documentoNuevo = os.path.isfile(file)

   if documentoNuevo is True:
       QMessageBox.information(None, "ERROR", "El nombre {} ya EXISTE!".format(self.dlg.nombreArchivo.text()))
   else:
    return False

And here is where it runs

def run(self):
    """
   QMessageBox.information(None, "valores", ",".join([str(x[0]) for x in cursorLista]))
   QMessageBox.information(None, "valores", "idLote={}".format(self.dlg.lote.text()))

    """
    self.dlg.lote.clear()
    self.dlg.referencia.clear()
    self.dlg.nombreArchivo.clear()
    self.dlg.orden.clear()
    self.dlg.comboBox.clear()
    self.dlg.desLote.clear()

    self.dlg.lote.textChanged.connect(self.onTextChanged)
    self.dlg.lote.textChanged.connect(self.buscaDescripcion)

    ix = self.dlg.comboBox.currentIndex()
    idSeccion = self.dlg.comboBox.itemData(ix)

    self.dlg.show()

    # Run the dialog event loop

    result = self.dlg.exec_()

    # See if OK was pressed
    if result:

        #Guarda imagen en servidor

        rutaArchivo = "D:/temp/"+"Lote-"+self.dlg.lote.text()+"_"+self.dlg.nombreArchivo.text()+".png"

        resulArchivo = self.verificaArchivo(rutaArchivo)

        if resulArchivo is False:

            self.iface.mapCanvas().saveAsImage(rutaArchivo)
            self.insertaInformes(self.dlg.lote.text(),self.dlg.referencia.toPlainText(),self.dlg.nombreArchivo.text()+".png",idSeccion,self.dlg.orden.text())

        pass

How can I achieve that if the function returns True, allow the user to modify the name of the document in self.dlg.filename.text ()

class SaveInformesDialog(QtWidgets.QDialog, FORM_CLASS):
    def __init__(self, parent=None):
        """Constructor."""
        super(SaveInformesDialog, self).__init__(parent)
        # Set up the user interface from Designer.
        # After setupUI you can access any designer object by doing
        # self.<objectname>, and you can use autoconnect slots - see
        # http://qt-project.org/doc/qt-4.8/designer-using-a-ui-file.html
        # #widgets-and-dialogs-with-auto-connect
        self.setupUi(self)

Code

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>SaveInformesDialogBase</class>
 <widget class="QDialog" name="SaveInformesDialogBase">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>516</width>
    <height>355</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>GuardaInformes</string>
  </property>
  <widget class="QDialogButtonBox" name="button_box">
   <property name="geometry">
    <rect>
     <x>330</x>
     <y>310</y>
     <width>171</width>
     <height>32</height>
    </rect>
   </property>
   <property name="orientation">
    <enum>Qt::Horizontal</enum>
   </property>
   <property name="standardButtons">
    <set>QDialogButtonBox::Cancel|QDialogButtonBox::Ok</set>
   </property>
  </widget>
  <widget class="QLabel" name="label">
   <property name="geometry">
    <rect>
     <x>10</x>
     <y>90</y>
     <width>121</width>
     <height>21</height>
    </rect>
   </property>
   <property name="font">
    <font>
     <pointsize>12</pointsize>
    </font>
   </property>
   <property name="text">
    <string>Referencia</string>
   </property>
  </widget>
  <widget class="QLineEdit" name="orden">
   <property name="geometry">
    <rect>
     <x>150</x>
     <y>230</y>
     <width>81</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QLabel" name="label_2">
   <property name="geometry">
    <rect>
     <x>10</x>
     <y>10</y>
     <width>121</width>
     <height>21</height>
    </rect>
   </property>
   <property name="font">
    <font>
     <pointsize>12</pointsize>
    </font>
   </property>
   <property name="text">
    <string>Numero Lote</string>
   </property>
  </widget>
  <widget class="QLabel" name="label_3">
   <property name="geometry">
    <rect>
     <x>10</x>
     <y>230</y>
     <width>81</width>
     <height>21</height>
    </rect>
   </property>
   <property name="font">
    <font>
     <pointsize>12</pointsize>
    </font>
   </property>
   <property name="text">
    <string>Orden</string>
   </property>
  </widget>
  <widget class="QLabel" name="label_4">
   <property name="geometry">
    <rect>
     <x>10</x>
     <y>50</y>
     <width>121</width>
     <height>21</height>
    </rect>
   </property>
   <property name="font">
    <font>
     <pointsize>12</pointsize>
    </font>
   </property>
   <property name="text">
    <string>Seccion</string>
   </property>
  </widget>
  <widget class="QLineEdit" name="nombreArchivo">
   <property name="geometry">
    <rect>
     <x>150</x>
     <y>270</y>
     <width>351</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QLineEdit" name="lote">
   <property name="geometry">
    <rect>
     <x>150</x>
     <y>10</y>
     <width>81</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QComboBox" name="comboBox">
   <property name="geometry">
    <rect>
     <x>150</x>
     <y>50</y>
     <width>351</width>
     <height>22</height>
    </rect>
   </property>
  </widget>
  <widget class="QLabel" name="label_6">
   <property name="geometry">
    <rect>
     <x>10</x>
     <y>270</y>
     <width>121</width>
     <height>21</height>
    </rect>
   </property>
   <property name="font">
    <font>
     <pointsize>12</pointsize>
    </font>
   </property>
   <property name="text">
    <string>Nombre Archivo</string>
   </property>
  </widget>
  <widget class="QTextEdit" name="referencia">
   <property name="geometry">
    <rect>
     <x>150</x>
     <y>90</y>
     <width>351</width>
     <height>121</height>
    </rect>
   </property>
  </widget>
  <widget class="QLineEdit" name="desLote">
   <property name="geometry">
    <rect>
     <x>240</x>
     <y>10</y>
     <width>261</width>
     <height>20</height>
    </rect>
   </property>
  </widget>
 </widget>
 <resources/>
 <connections>
  <connection>
   <sender>button_box</sender>
   <signal>accepted()</signal>
   <receiver>SaveInformesDialogBase</receiver>
   <slot>accept()</slot>
   <hints>
    <hint type="sourcelabel">
     <x>20</x>
     <y>20</y>
    </hint>
    <hint type="destinationlabel">
     <x>20</x>
     <y>20</y>
    </hint>
   </hints>
  </connection>
  <connection>
   <sender>button_box</sender>
   <signal>rejected()</signal>
   <receiver>SaveInformesDialogBase</receiver>
   <slot>reject()</slot>
   <hints>
    <hint type="sourcelabel">
     <x>20</x>
     <y>20</y>
    </hint>
    <hint type="destinationlabel">
     <x>20</x>
     <y>20</y>
    </hint>
   </hints>
  </connection>
 </connections>
</ui>