Allow `purpose.match` to supply a reason it did not match

[aljones15]

jsonld-signatures/lib/ProofSet.js

Lines 318 to 321 in 9a665c5

	// first check if the proof matches the purpose; if it doesn't continue
	if(!await purpose.match(proof, {document, documentLoader, expansionMap})) {
	continue;
	}

if a purpose does not match no Error or Reason is given. Then if no purposes match at all then this generic error is thrown:

jsonld-signatures/lib/ProofSet.js

Lines 151 to 157 in 9a665c5

	if(results.length === 0) {
	const error = new Error(
	'Did not verify any proofs; insufficient proofs matched the ' +
	'acceptable suite(s) and required purpose(s).');
	error.name = 'NotFoundError';
	throw error;
	}

jsigs should allow purpose.match to supply a reason for not matching which is reported back to the user.

[dmitrizagidulin]

+1, this would be a great feature.