improve exception naming

[abacha]

All the errors raise the same exception "Mysql2::Error", it would be nice if we have specific exceptions for each error like: "invalid query", "connection closed" etc
so it could be handled on the code

[brianmario]

I definitely plan on adding specific exceptions for all of the different error types MySQL can return. Just need to find the time to do it (out of town currently). I was planning on going through the list of error codes we can get back from MySQL (via http://dev.mysql.com/doc/refman/5.5/en/error-messages-server.html ?) and create exception classes which extend from Mysql2::Error for all of them or at least the ones that make sense.

Then I can centralize the error handling in Mysql2 itself so that these new exception classes would be raised for those specific error codes. It shouldn't break anyone currently rescuing from Mysql2::Error since these new classes would extend from it.

[brianmario]

Just FYI - I'm still planning on doing this but it won't be in 0.3.12 which I'm hoping to release within a couple of days.

[simi]

duplicate of #404

[sodabrew]

I think #911 makes good progress. Closing as done!