Add modern API error types and update tests for Djblets error types.
Review Request #13264 — Created Sept. 6, 2023 and submitted
Djblets 4 is gaining additional support in API error payloads for error
type strings (a more useful equivalent to numeric codes) and other
useful error information.
This change adds types to Review Board's API errors, and documents the
errors in the process. It also updates unit tests to check against the
updated payloads with type information.
Unit tests pass.