PSET: VerifyBlindValueProof: fail verification if the value or asset commitment is missing
PSET: VerifyBlindValueProof: fail verification if the value or asset commitment is missing

It is arguable whether attempting a value proof without an asset
commitment should be treated as incomplete or incorrect blinding. Since
the asset commitment is required to produce the value proof in the first
place, failing to provide it in the source PSET and thus at this call
site is treated as an error.

This does not change the existing behaviour since a missing asset
or value commitment in this function would already cause it to fail.

Future work on the blinding iterface should likely return the blinding
state directly from the verify calls and remove the duplicated mapping of
blinding error codes to transaction blinding error codes, instead of the
verify functions returning just true/false.

ElementsProject/elements,ElementsProject/elements,ElementsProject/elements,ElementsProject/elements,ElementsProject/elements,ElementsProject/elements