Factor issue and MR edit form error list.

Direct continuation to https://github.com/gitlabhq/gitlabhq/pull/7678

This one factors the error messages.

Note: I am unable to view the error messages, because the only thing that could generate an error, an empty title, is not allowed by the browser because of the "required: true", so I could not test it the CSS looks good.

If anyone knows of a way to do this test, please tell me.

In any case, that is basically dead code, and now it is factored dead code.