Re #13616 More descriptive error message when properties fail to load

On attempting a sequential fit, if the properties cannot be loaded, a
possible cause is that they have been cleared by changing the instrument.
Tell the user this, show them what the original error was and display
the error in the log too.

MantidQt/CustomInterfaces/src/Muon/MuonAnalysis.cpp

@@ -3124,6 +3124,16 @@ void MuonAnalysis::openSequentialFitDialog()
   {
     loadAlg = createLoadAlgorithm();
   }
+  catch (const std::runtime_error &err) {
+    QString message("Error while setting load properties.\n"
+                    "If instrument was changed, properties will have been "
+                    "cleared and should be reset.\n\n"
+                    "Error was: ");
+    message.append(err.what());
+    QMessageBox::critical(this, "Unable to open dialog", message);
+    g_log.error(message.ascii());
+    return;
+  }
   catch(...)
   {
     QMessageBox::critical(this, "Unable to open dialog", "Error while setting load properties");