Better error message / pre-emptive checking for whether the user has appropriate permissions to add a repo webhook

[mightybyte]

In GitLab by @mightybyte on Jul 1, 2019, 17:17

If your account doesn't have appropriate permissions to create webhook for a particular repo, adding the repo will fail with an inscrutable error message. We need to either return a better error message or do some checking before the add to tell the user that they don't have access.