rabbit_queue_location_validator: Fix pattern in validate_policy/1

validate_strategy/1 error value is `{error, FormatString, Args}`, not
the usual `{error, Reason}`.

This incorrect pattern matching was found thanks to the
`set_policy_command_test.exs` testsuite in rabbitmq-cli.

(cherry picked from commit 3976d87)

Author: dumbbell

src/rabbit_queue_location_validator.erl

@@ -33,7 +33,7 @@
 validate_policy(KeyList) ->
     case proplists:lookup(<<"queue-master-locator">> , KeyList) of
         {_, Strategy} -> case validate_strategy(Strategy) of
-                             {error, _} = Er -> Er;
+                             {error, _, _} = Er -> Er;
                              _ -> ok
                          end;
         _             -> {error, "queue-master-locator undefined"}