Provide better message when merge fails due to file locking
This happened on gitlab-com/www-gitlab-com!11691 (merged). While I was trying to merge the MR, I was told that the pre-receive hook failed with no other information provided. If someone hadn't seen the production logs, we wouldn't know what the real error was gitlab-com/www-gitlab-com!11691 (comment 80083722):
MergeService ERROR: gitlab-com/www-gitlab-com!11691 - GitLab: The path 'source/posts/2011-11-07-new-design-is-coming.html.md' is locked by Emily<br>
MergeService ERROR: gitlab-com/www-gitlab-com!11691 - Something went wrong during merge pre-receive hook