Decide on AppError codes

[joshua-kim]

Seems like the app error codes are a bit inconsistent rn. Will take this convo out of this PR.

Originally posted by @StephenButtolph in #3434 (comment)

AppError is somewhat inconsistent - originally the idea was to have a way to define errors that likely needed client-side behavior to occur (ErrThrottled -> backoff), but some errors exist (i.e ErrNotValidator, ErrUnregisteredHandler) that don't.

[github-actions]

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