Improve error reporting from VerifyAmounts

[stevenroose]

When running this, I get a segmentation fault, so there is something wrong with the references or something.. borrowck, plz help me..

[delta1]

What's your input? You're getting a segfault when running SignPSBT with this patch?

[apoelstra]

I think if we're going to be touching core consensus code like this we should do it properly, with an error enum.