When a consensus doesn't have enough signatures, write it (and sigs) to a file

Just like we write 'unparseable-desc' and 'v3-status-votes' to a file, it would be great to write 'unpublished[-flavour]-consensus' to a file, when we log this message:

Nov 10 21:00:01.000 [warn] A consensus needs 5 good signatures from recognized authorities for us to accept it. This one has 4 (...). 4 (...) of the authorities we know didn't sign it.
Nov 10 21:00:01.000 [warn] Not enough info to publish pending ns consensus

Similarly, when we disagree about signatures, we should write 'unpublished-[-flavour]-signatures', when we log this message:

Nov 10 21:27:32.000 [warn] Problem adding detached signatures from <redacted-address-and-port>: Mismatched digest.

(Or, even better, write both matching and mismatching signatures to the file.)

One catch: These new filenames need to be added to the sandbox.

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