Implement more specific exception classes

[brianmario]

These should all inherit from Mysql2::Error as the base and should probably each map to one of the codes listed here. Unsure if we need to worry about these as well. Will have to look into that when it comes time.

[simi]

@brianmario yes we need both. Do you have any plan how to implement this? Define all error codes as exceptions with error code and find them by error code and raise with error message?

If you'll point me I can try to do this.