Improve logging for enable_root_user not enabled instead of "Authentication failed for user

When a user is considered a fully privileges user (not sure how the matching is done) and enable_root_user is disabled, the error message returned is a plain:

warning: (9) [MariaDBProtocol] Authentication failed for user 'claudio'@[192.168.1.135] to service 'Read-Write-Service'. Originating listener: 'Read-Write-Listener'. MariaDB error: 'Access denied for user 'claudio'@'192.168.1.135' (using password: YES)'.

The user of course was perfectly able to login directly to the backends.

It took me some time to find out that 'claudio'@'%' had too many privileges and was considered as 'root' and so denied login.

I think it would be useful to log a more meaningful message like:

Fully privileges users are not allowed to login set enable_root_user=1 if needed.