When fast-forward merge is enabled, merges might fail due to a failed
pre-receive check (e.g. due to file locks, commit message push rules,
etc.) but the UI would only display:

```
Merge failed: pre-receive hook failed. Please try again.
```

While the raw message was logged in Sentry and in `exceptions_json.log`,
this kept users in the dark.

Gitaly prefaces the error response status from the `/internal/allowed`
API call with `GitLab:`, so we can use this to display the error to the
user. We now only fallback to a default if we do not have anything to
show. Now, users see a more informative message, such as:

```
Merge failed: Commit message does not follow the pattern
'MERGEME'. Please try again.
```

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/246816