More descriptive message for git hooks and file locks

What does this MR do?

Most of the benefits will be in GitLab EE because we have git hooks there (push rules) and file lock features. Instead of showing "Git operation was rejected by #{hook_name} hook" the actual problem description will be shown. Also you will see a clear message when you edit locked file.

Why was this MR needed?

Because we don't want to confuse our users

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/713

@vsizov looks good!

Enabled an automatic merge when the build for bf218337 succeeds

Status changed to merged

mentioned in commit aa9b97f0

mentioned in issue #19190 (closed)

mentioned in issue #17766 (closed)

mentioned in issue #19439 (closed)