There are a number of special cases (and inconsistencies with wallet.cpp)
in the blinding logic that can prevent blinding from succeeding when various
intermediate values are zero. This fixes two of them.
A proper fix would involve adding new APIs to secp-zkp and then overhauling
all of the wallet blinding code. But for now let's get the tests working.
A couple issues with this -- one is that it was verifying the asset
commitment against itself, rather than against the explicit asset,
and the other is that the verification logic had an extra `== 0` at
the end which inverted the verification check.
Both pretty embarassing to have not caught in review..
Instead of not failing and silently allowing bad PSETs to be created,
provide error messages that explain why the operation the user did was
wrong. Specifically for combining PSETs such that they would become
fully blinded but have imbalanced values and blinders, and for blinding
a PSET when the wallet has a blind input but no outputs to blind (would
result in an imbalance).
