Expose error mnemonics on contract metadata

[utdemir]

TZIP-16 supports exposing a mapping from error ids to human-readable messages. That might be useful for the clients, and should be easy for us to implement since we already have the mapping (our errors.ml).

However, I do not know if there are any clients/wallets making use of this feature; so probably this is not a priority.

[purcell]

Ooh, nice find. Agree, not high priority, but it certainly could be helpful. If we add it we should mention it to the Cogarius folks.