Merge branch 'fix-diff-flash' into 'master'

only show error if there is an error

See merge request gitlab-org/gitlab-ce!17228

app/assets/javascripts/single_file_diff.js

@@ -88,6 +88,8 @@ export default class SingleFileDiff {
 
         if (cb) cb();
       })
-      .catch(createFlash(__('An error occurred while retrieving diff')));
+      .catch(() => {
+        createFlash(__('An error occurred while retrieving diff'));
+      });
   }
 }