Remove HY000 from database failure states:

* Reverts changes in cc30c87c7bd98a1b92e957cc5ccabc40616b429b
* HY000 is a very generic error that some databases or drivers will return it for any error that has no specific sql state, such as mysql
* the entire connection pool should not be killed when this code refers to benign errors
* mysql docs: Not all MySQL error numbers have corresponding SQLSTATE values. In these cases, 'HY000' (general error) is used.
* HY000 was originally added for firebird in 37b3b7c85361f259ef6fd01a5eaec9341e937b91
Merge pull request #20 from matt-blanchette/master

Remove HY000 from database failure states: