deploy_gitlab_ci: show instructions if merge fails

deploy_gitlab_ci.py

@@ -107,7 +107,13 @@ def main():
     merged = accept_merge_request(project, mr, rebase=True)
 
     if not merged:
-        sys.exit("Integration MR could not be merged")
+        sys.exit(
+            "Integration MR could not be merged. You have two possibilities to fix "
+            "this:\n"
+            "  1. Checkout the MR and rebase it on the current master manually, or\n"
+            "  2. Delete the MR (Edit -> Delete in the MR UI)\n"
+            "In either case restart this job afterwards in order to get it merged."
+        )
 
     print("Successfully merged")