Feature: Include error codes in error responses.

[mrfelton]

Feature Description

We should include error codes in the lndk error responses. Currently we can do parsing of error message strings (e.g. the ones from here), but the confidence level that error messages won't be changed over time is less than it would be for error codes.

Design

Ideally, error responses should include both a code and a message. Message to convey human readable detail about the problem. Code to enable robust machine processing.