Consider improving user feedback when an ajax request 500's
On https://gitlab.com/gitlab-org/gitlab-ce/issues/49061 when the backend returned a 500 error the frontend simply kept showing a spinner instead of letting the user that something went wrong. This can cause our users frustration since they don't know what's going on. We should consider having better feedback when errors like this occur.
cc/ @janszumiec as he originally suggested we improve this (thanks!)