Fix app quitting after failed save. Fixes #4098

Return `false` from Document::save() to properly communicate the
failure, which the app-quit process already handles by aborting.

src/Gui/Document.cpp

@@ -1161,6 +1161,7 @@ bool Document::save(void)
         catch (const Base::Exception& e) {
             QMessageBox::critical(getMainWindow(), QObject::tr("Saving document failed"),
                 QString::fromLatin1(e.what()));
+            return false;
         }
         return true;
     }