Merge branch 'git_hooks_better_message' into 'master'

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


See merge request !5067