refactor(log): show merge request web url when it fails to assign a reviewer

this url may be useful for debugging and eventual fixing permission.

assignbot/__main__.py

@@ -126,7 +126,10 @@ class ReviewAssigner:
                 query_data={"assignee_id": user.id},
             )
         except GitlabHttpError:
-            print(f"ERROR: no permission to assign a reviewer to {merge_request.title}")
+            print(
+                f"ERROR: no permission to assign a reviewer to {merge_request.title}"
+                f" ({merge_request.web_url})"
+            )
             return
 
         self.review_states.loc[len(self.review_states)] = (