mrhlpr: Link documentation for how to set up personal access token on error

Previously you'd just get an error that GITLAB_TOKEN wasn't set in the
environment without any information about how to resolve this.

mrhlpr/mr.py

@@ -449,6 +449,14 @@ def finish(mr_id: int, comment: str | None, no_cache: bool = False):
         logging.error("python-gitlab is needed to run 'finish'")
         exit(1)
 
+    gitlab_token = os.environ.get("GITLAB_TOKEN")
+
+    if not gitlab_token:
+        logging.error(
+            "To use 'finish' you need a personal access token set in the environment as GITLAB_TOKEN. See https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html."
+        )
+        exit(1)
+
     print("Pushing changes...")
     status = get_status(mr_id, no_cache)
     origin = gitlab.parse_git_origin()
@@ -457,7 +465,7 @@ def finish(mr_id: int, comment: str | None, no_cache: bool = False):
         remote_local = "origin"
     git.push_head(remote_local, status.source_branch)
 
-    gl = gitlab_api.Gitlab(url="https://gitlab.com", private_token=os.environ["GITLAB_TOKEN"])
+    gl = gitlab_api.Gitlab(url="https://gitlab.com", private_token=gitlab_token)
     project = gl.projects.get(origin.project_id)
     mr = project.mergerequests.get(mr_id)
     print("Approving MR...")