Generic agent side error handling over GRPC
As suggested in the review of !43 (merged), we need a way for generic error handling via GRPC. We seem to need some new structure in the agent.proto which will describe errors, similar to this:
https://cloud.google.com/apis/design/errors#error_model
For the time being, it may simply be a structure with a single field comprising an error message in the textual format.