Merge branch 'validate-only-except-regexp' into 'master'

Validate only and except regexp

## What does this MR do?
Adds a better validation for only and except which can contain regexps.

## Why was this MR needed?
Currently the RegexpError can be raised when processing next stage which leads to 500 in different places of code base.
This adds early check that regexps used in only and except are valid.

cc @grzesiek 

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4736