[Protocol] Emit failure reason from the connector

[cgardens]

Tell us about the problem you're trying to solve

@pmossman exposed the source of sync failure down to the granularity of source, worker, destination, etc. The next step here is, in the case of a connector failure, to have the connector report what the problem is. There are two direct things that users want here.

1. Error type - the connector should be able to emit whether the failure is due to: user configuration error, transient error, unknown (assumes bug). Being able to expose this enum gives us lots of options to improve the user experience. We can alert user when they need to update a configuration.
2. User readable error - right now users have to trawl through the logs to discover why a connector failed. many users can't or won't do this. instead, where possible we should provide an actionable, user readable error message. This message can then be displayed by the platform.

Describe the solution you’d like

• This feature requires some change to the protocol to enable this functionality.
• Whatever approach we take here must be possible to roll out incrementally to connectors.

[cgardens]

Closed as duplicate of #12161