Specify chain index in gq error

Author: amas0

cmdstanpy/stanfit/gq.py

@@ -635,6 +635,7 @@ def _assemble_generated_quantities(self) -> None:
             except Exception as exc:
                 raise ValueError(
                     f"An error occurred when parsing Stan csv {csv_file}"
+                    f" for chain {chain}"
                 ) from exc
         self._draws = gq_sample