Return error 500, if possible, from GET /info/refs

Closes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/121

@jacobvosmaer-gitlab Thanks for the quick turnaround!

Is there a reason we don't have a 9.0 milestone? I suppose release managers don't have to pick this into stable; just the version bump is needed.

@stanhu no reason why there is no 9.0 milestone but I'll make one.

Yes, the crucial part is the GITLAB_WORKHORSE_VERSION bump in gitlab-ce once this is reviewed, merged, released.

changed milestone to %9.0

merged

mentioned in commit tmaczukin/gitlab-workhorse@ce2c92f8

mentioned in issue omnibus-gitlab#2119 (closed)