Report permissions failures with more detail.

[tmcgilchrist]

Currently when the gitlab token is missing the correct permissions. It will throw this error:

{"state":"failure","name":"ocaml-ci","description":"debian-12-4.12_opam-2.2 failed: Failed: Build failed","target_url":"https://ocaml.ci.dev/gitlab/raphael-proust/entombed/commit/d81eac06b41cddfe537acedd2f1ff441cae3ea34"}
2024-10-18 21:26.14: Job failed: Failed to set GitLab status Gitlab_core.Make(Env)(Time)(CL).Message(-438114087, _)

It should be possible to include details on the project / user plus the underlying GitLab error.