Treat Merge Request errors consistently

We have a field merge_requests.merge_error which is used for storing static (human-defined) errors like https://gitlab.com/gitlab-org/gitlab-ee/blob/6dc2e23c910b5847de8b1f7ef6fa2d531a105b8a/app/services/merge_requests/merge_service.rb#L54 and sometimes we're writing a raw error message like here https://gitlab.com/gitlab-org/gitlab-ee/blob/6dc2e23c910b5847de8b1f7ef6fa2d531a105b8a/app/services/merge_requests/merge_service.rb#L86-89

let's be consistent and not write raw errors to that field.

Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16326