Feature request: allow tool module to specify reason of unknown result

[TDacik]

If I understand correctly, the tool module can specify reason for an error result by using RESULT_ERROR + " (error)", but this is not possible for RESULT_UNKNOWN (RESULT_UNKNOWN + " (incomplete)" is classified as an error). It is of course possible to use just errors, but I think it is useful to distinguish between them. Would it be possible to support this functionality for unknown results as well?

[PhilippWendler]

I guess that should be doable. I would that to think through the consequences so I can't do it right now, though.