prompt the user to load backup when load project fails

If an error occurs while loading the project file, prompt the user to load the backup file instead, if it exists.

src/app/qgisapp.cpp

@@ -4312,13 +4312,35 @@ bool QgisApp::addProject( const QString& projectFile )
 
   if ( !QgsProject::instance()->read( projectFile ) )
   {
+    QString backupFile = projectFile + "~";
+    QString loadBackupPrompt;
+    QMessageBox::StandardButtons buttons;
+    if ( QFile( backupFile ).exists() )
+    {
+      loadBackupPrompt = "\n\n" + tr( "Do you want to open the backup file\n%1\ninstead?" ).arg( backupFile );
+      buttons |= QMessageBox::Yes;
+      buttons |= QMessageBox::No;
+    }
+    else
+    {
+      buttons |= QMessageBox::Ok;
+    }
     QApplication::restoreOverrideCursor();
     statusBar()->clearMessage();
 
-    QMessageBox::critical( this,
-                           tr( "Unable to open project" ),
-                           QgsProject::instance()->error() );
+    int r = QMessageBox::critical( this,
+                                   tr( "Unable to open project" ),
+                                   QgsProject::instance()->error() + loadBackupPrompt,
+                                   buttons );
 
+    if ( QMessageBox::Yes == r && addProject( backupFile ) )
+    {
+      // We loaded data from the backup file, but we pretend to work on the original project file.
+      QgsProject::instance()->setFileName( projectFile );
+      QgsProject::instance()->setDirty( true );
+      mProjectLastModified = pfi.lastModified();
+      return true;
+    }
 
     mMapCanvas->freeze( false );
     mMapCanvas->refresh();