Show error message on incorrect validation

Refs #13654

Code/Mantid/MantidQt/CustomInterfaces/src/Indirect/ISISEnergyTransfer.cpp

@@ -69,18 +69,21 @@ bool ISISEnergyTransfer::validate() {
   UserInputValidator uiv;
 
   // Run files input
-  if (!m_uiForm.dsRunFiles->isValid())
+  if (!m_uiForm.dsRunFiles->isValid()){
     uiv.addErrorMessage("Run file range is invalid.");
+  }
 
   // Calibration file input
   if (m_uiForm.ckUseCalib->isChecked() &&
-      !m_uiForm.dsCalibrationFile->isValid())
+      !m_uiForm.dsCalibrationFile->isValid()){
     uiv.addErrorMessage("Calibration file/workspace is invalid.");
+  }
 
   // Mapping file
   if ((m_uiForm.cbGroupingOptions->currentText() == "File") &&
-      (!m_uiForm.dsMapFile->isValid()))
+      (!m_uiForm.dsMapFile->isValid())){
     uiv.addErrorMessage("Mapping file is invalid.");
+  }
 
   // Rebinning
   if (!m_uiForm.ckDoNotRebin->isChecked()) {
@@ -102,6 +105,9 @@ bool ISISEnergyTransfer::validate() {
     m_uiForm.valRebinString->setVisible(false);
   }
 
+  QString error = uiv.generateErrorMessage();
+  showMessageBox(error);
+
   return uiv.isAllInputValid();
 }