Initial implementation of API errors data structure
Allow Errors to be an error itself

This has Errors implement the error interface, allowing it to pose as an error
itself. Use of this in the server may be minimal, for now, but it's useful for
passing around opaque client errors.

A method, PushErr, has also been add to allow arbitrary errors to be passed
into the Errors list. This keeps the errors list flexible, allowing the app to
collect and errors before we have codes properly mapped.
Merge pull request #711 from stevvooe/errors-marshaling

Initial implementation of API errors data structure