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

org-sync/org-sync-gitlab.el

Line 176 in 88aa037

(json-read)

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!!)