Added an exception hierarchy shadowing DB-API exc types

Added an exception hierarchy shadowing DB-API exc types
No more generic SQLErrors wrappers- the shadow type matching the DB-API error is raised. [ticket:706]
SQLError is now (also) DBAPIError.
DBAPIError and subtype constructors will refuse to wrap a SystemExit or KeyboardInterrupt, returningthe original interrupt exception instead of a new instance. [ticket:689]
Added a passthroughs for SE/KI exceptions in a couple except-and-discard situations