rgw: quiet down warning message

Fixes: #6123
We don't want to know about failing to read region map info
if it's not found, only if failed on some other error. In
any case it's just a warning.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>