Refactor the client error handling in libdrizzle

Figuring out what to do with the ER() macros and client_errors[].

This whole thing is a mess, as there are global static arrays that are shared between client and server, and we generate these things with comp_err. I'd say it's a later design issue to be solved, but it's keeping libdrizzle tied to mysys... and it's a spiderweb anyway - MTaylor

I should also mention that there are a couple of places where the IMPL
code of libdrizzle references mysqld errors (ER_*) as a protocol
library, this is insane... either the error codes are ones defined by
the protocol (in which case they should be defined by the lib) or they
are internal mysqld errors, in which case the client should never see them. - MTaylor