Improve error messages when file editing fails

Give more specific errors in API responses and web UI flash messages when a file update fails.  See #1479.

Instead of returning false from `Gitlab::Satellite::Files::EditFileAction#commit!` when a `Grit::Git::CommandFailed` error is raised, now `#commit!` raises a different error depending on whether the failure happened during checkout, commit, or push.

@dzaporozhets Please let me know if you want to change the HTTP status codes or the error messages in `Files::UpdateService`

cc @sytse

See merge request !1569