Error messages from custom pre-receive hooks now appear in the GitLab
UI.

This is re-enabling a feature that had been disabled in merge request
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18646

The feature had been disabled due to security concerns that information
which was not intended to be public (like stack traces) would leak into
public view.

PreReceiveErrors (from pre-receive, post-receive and update custom
hooks) are now filtered for messages that have been prefixed in a
particular way.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48132