MySQL2 gem doesn't handle new MariaDB10.4 error code: 1396

[Rudis1261]

Hi There,

Seems that the gem doesn't handle new error codes introduced by MariaDB 10.4. See this following error code 1396 which MariaDB throws when attempting to insert a new user which collides with an existing user:

ERROR 1396 (HY000): Operation CREATE USER failed for 'user4e63'@'%'

I did grep through the code and it seems this error code isn't currently handled. It would be awesome if it could catch this error code, and re-throw it in a more descriptive way. So clients are able to make a decision based on that result.

The error code is documented here: https://mariadb.com/kb/en/mariadb-error-codes/

Error Code	SQLSTATE	Error	Description
1396	HY000	ER_CANNOT_USER	Operation %s failed for %s

There may be more that aren't currently accounted for.