rgw_rest_admin: return -EINVAL for improper user info requests

rgw_rest_admin: return -EINVAL for improper user info requests

for /admin/user GET requests, if a uid is not given, we ultimately
return an AccessDenied error, as we initialize an anon user by default
where operations like key initialization will throw an -EACCESS. Since
the actual failure here is that uid was not specified, return an
-EINVAL, HTTP 400 error instead which is more clearer.

Fixes: http://tracker.ceph.com/issues/15455
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>