Always log Err as error! is too severe.

[wuerges]

Currently, the mysql_async crate is emitting all Err variants as error! events.

Most of the time this is desirable, but sometimes this level is too severe.

When inserting duplicate values, deleting a value that is in use by another table, matching the Err variant is very convenient. A query that returns Err could be a benign error that is handled by the application, and a lower event level would be better.

This PR helps explain the issue, and attempts to solve it:
#319