Improve error message on collapse due to intersection avoidance

src/app/qgsmaptooladdfeature.cpp

@@ -268,7 +268,11 @@ void QgsMapToolAddFeature::canvasReleaseEvent( QMouseEvent * e )
 
         if ( !f->geometry()->asWkb() ) //avoid intersection might have removed the whole geometry
         {
-          QMessageBox::critical( 0, tr( "Error" ), tr( "The feature cannot be added because it contains an emtpy geometry" ) );
+          QString reason = tr( "The feature cannot be added because it contains an empty geometry" );
+          if ( avoidIntersectionsReturn == 2 ) {
+            reason += QString( " (" + tr("collapsed due to intersection avoidance") +  ")" );
+          }
+          QMessageBox::critical( 0, tr( "Error" ), reason );
           delete f;
           stopCapturing();
           return;