GitLab backend needs to do error handling

[plajjan]

When I ran into #41 but before I knew what caused it all I got from Emacs/org-sync was: helm-M-x: JSON readtable error which isn't particularly helpful at all. I think it originates from this https://github.com/arbox/org-sync/blob/88aa037ef0f5d6f3fe8a5730d67baeb6e126a794/org-sync-gitlab.el#L176

As I wrote in #41, we're actually getting a 414 error back from the server, so how about we check the HTTP status code before we try to JSON parse the message? It should always be 200 OK I guess or we can throw an error (but about the HTTP status code and not JSON!!)