Bubble up error detail in gRPC service

Currently the gRPC service relies on `ProcessImpl` for performing executions, and that will result in a boolean to indicate success/failure.
However, for the gRPC service it would be very useful to be able to be more specific when an execution is marked as a failure (especially when the termination predicate feature lands). Bubble up the error message so it can be forwarded in the error detail of gRPC replies.