Fix Session._set_keyspace_for_all_pools to report all pools' errors

The final callback was invoked with host_errors (the errors from only
the last pool to finish) instead of the accumulated errors dict. If the
last pool succeeded, failures from other pools were silently lost. Pass
the aggregated errors dict, matching the method's docstring.

Author: sylwiaszunejko

cassandra/cluster.py

@@ -3438,7 +3438,7 @@ def pool_finished_setting_keyspace(pool, host_errors):
                 errors[pool.host] = host_errors
 
             if not remaining_callbacks:
-                callback(host_errors)
+                callback(errors)
 
         for pool in tuple(self._pools.values()):
             pool._set_keyspace_for_all_conns(keyspace, pool_finished_setting_keyspace)