Change the proof verification functions to return errors instead of bools

[kcalvinalvin]

It's hard to tell what the actual errors were because:

1: Error messages are vague
2: Some functions drop some details about errors (ex. IngestBatchProof() in pollardproof.go)

Changing these functions will help debugging easier

[adiabat]

Yup, I started doing this in my hashtoknown branch. Not everything yet but definitely agree.