Return more information if ShareProof and RowProof verification failed

[rach-id]

          [separate issue] since `VerifyProof` only returns a bool value, I expect callers will be need debugging tools if they receive a `false` here.

We could consider modifying the API so that an error is returned if VerifyProof returns false.

// VerifyProof returns true if ...
// VerifyProof returns false and an error if ...
func (sp ShareProof) VerifyProof(root []byte) (bool, error) {

Originally posted by @rootulp in #1357 (comment)