Exception cleanup

Exceptions are all over the place: Some are in the api package (from the idea they are part of the public api), others of them are in internal packages (from the idea that why bother the api user w/ a bunch of exception definitions when they are looking at the class structure?).

Logically, it seems like exceptions are part of the API b/c the user will potentially see them, but the implementations and the many subclasses are not really part of the API. So, perhaps a rule of thumb would be: **Top level Exception types are public APIs, specific subclasses are not.**

The current pattern is a top level class w/ lots of inner subclasses for specific types. This doesn't translate well to this new guideline, so perhaps splitting these into an interface and implementations would simplify?