gitlab_gql: print error returned by server in --print-merged-yaml

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27535>

bin/ci/gitlab_gql.py

@@ -343,8 +343,12 @@ def fetch_merged_yaml(gl_gql: GitlabGQL, params) -> dict[str, Any]:
       - local: .gitlab-ci.yml
     """)
     raw_response = gl_gql.query("job_details.gql", params)
-    if merged_yaml := raw_response["ciConfig"]["mergedYaml"]:
+    ci_config = raw_response["ciConfig"]
+    if merged_yaml := ci_config["mergedYaml"]:
         return yaml.safe_load(merged_yaml)
+    if "errors" in ci_config:
+        for error in ci_config["errors"]:
+            print(error)
 
     gl_gql.invalidate_query_cache()
     raise ValueError(