Add pool label to "resource requests ... are unsatisfiable" message (#…

…342)

Pool labels are a CPG-local addition (PR #197) that are considered
in select_inst_coll() so should be displayed in this error message.

batch/batch/front_end/front_end.py

@@ -1504,6 +1504,7 @@ async def _create_jobs(
                 f'memory={resources.get("req_memory")}, '
                 f'storage={resources["req_storage"]}, '
                 f'preemptible={preemptible}, '
+                f'pool_label={pool_label}, '
                 f'machine_type={machine_type}'
             )