Add function name to errors and logs

[ionut-arm]

Adding the name of the function that lead to a backend error to the log messages it generates and to the error returned to the client.

cc @ellerh

[wiktor-k]

@ionut-arm it seems this is largely OK to merge (after fixing conflicts). Is there anything else you'd like to add to it?

[ionut-arm]

@ionut-arm it seems this is largely OK to merge (after fixing conflicts). Is there anything else you'd like to add to it?

Ok, I've incorporated your feedback, I don't think I can easily avoid breaking the interface since we rely on enums for errors..