db: Include LastSyncError on external service

Created by: ryanslade

We now join with the external_service_sync_jobs table and include the error from the most recent sync of the external service.

Some thoughts:

I decided to include the error in the external service object itself so that we could get everything in one query. Perhaps it would be better to only query for it from the resolver on demand?

I also needed to add the field to the api.ExternalService type which seems unnecessary. Also, why do we actually need a separate type here?

The way we return the errors to the client is really messy with string concatenation. Maybe we should use this as an opportunity to include a 'Warnings' array instead and deprecate Warning?