When a pre-receive error happens in the fast-forward merge, the original
error is discarded, and only text prefaced with `GL-HOOK-ERR` is passed
through. However, this makes it difficult to know what went wrong.

To fix this, we:

1. Store the raw message to `Gitlab::Git::PreReceiveError`
2. Log an exception with the raw message

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/227876