Handle custom Git hook result in GitLab UI

## What does this MR do?

This MR fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19190
It allows to show messages from git custom hooks

## Screenshots (if relevant)

![joxi_screenshot_1467651998090](/uploads/7f814523c96712c07b4f4b52c843aab3/joxi_screenshot_1467651998090.png)

See merge request !5073