accept_merge_request: explicitly check if MR has merge conflicts

According to the documentation [1] we should get a 406 error from the
API if we try to merge a merge request which has a merge conflict. We
are experiencing a different behaviour, though, where we are getting a
405 error in this case. There is an open GitLab issue on this topic [2].

In our error handling we assumed that the reason for a 405 error is most
likely a required pipeline which is still running. This assumtion,
however, leads to wrong behaviour in cases where no pipeline is started
at all.

Fixing this now by explicitly checking for merge conflicts at the very
beginning of the error handling sequence.

[1]: https://docs.gitlab.com/ee/api/merge_requests.html#merge-a-merge-request
[2]: https://gitlab.com/gitlab-org/gitlab/-/issues/364102