mrhlpr.mr: Improve error message when authentication fails (MR 47)

mrhlpr/mr.py

@@ -469,7 +469,15 @@ def finish(
     git.push_head(remote_local, status.source_branch)
 
     gl = gitlab_api.Gitlab(url="https://gitlab.com", private_token=gitlab_token)
-    gl.auth()
+    try:
+        gl.auth()
+    except gitlab_api.exceptions.GitlabAuthenticationError as authentication_exception:
+        logging.error(
+            "Authenticating with the token from GITLAB_TOKEN in the environment produced the following error: %s\n\nSee https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html",
+            authentication_exception,
+        )
+        exit(1)
+
     project = gl.projects.get(origin.project_id)
     mr = project.mergerequests.get(mr_id)
     if skip_approve or (gl.user and mr.author and gl.user.id == mr.author.get("id")):