rgw: do not leak handler in get_handler() error path

If we fail to initialize, delete the handler.

Fixes (part of): #5949
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
librados: fix MWatchNotify leak

Do not leak the message if the watcher is not registered.  Also, simplify
this block.

Fixes (part of): #5949
Backport: dumpling, cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>