Command responses should carry status code

[SalvatoreDiGirolamo]

If a handler issues a buggy command (e.g., to an undefined command interface) then the command request gets lost and no response is sent back.

We should:

• (1) add an error bit to the command response;
• (2) have a "sink" command interface that replies with responses flagged as errors;
• (3) propagate success/fail status to the handler whenever it tests/waits for a command.

What happens if a handler issued a buggy command and then exits without checking? We should probably send an event to the host in this case. This would probably make (3) redundant because we shouldn't spend too much time into the handlers, e.g., checking for status codes: if sth goes wrong, it could be enough to signal this to the host via an event.