This updates API error codes to coincide with changes to the proposal. Mostly,
redundant error codes were merged and missing ones were added. The set in the
main errors.go file will flow back into the specification.
A test case has been added to ensure ErrorCodeUnknown is included in marshaled
json.
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.
