Move validation errors into a pkg

This is a precursor to fixing my other PR about error lists - That one is on hold until this or something like it goes in.

This makes ErrorList a public type, but does not implement the error interface on it (because I think we want an empty list to not be an error).  ErrorList.ToError() produces an error or nil if the list is empty.  I put it under api/errors, because that is what it is used for, and the Make* functions are sort of validation-centric.
Either be goan and use "errors" or "errs", or be C++ish and say "api_validation_errors" :)

This is why imports can rename packages - one of the go docs says "don't worry about package name collisions, users can rename them on import" (which makes for awesome ungreppable code)