Return a list of exceptions encountered during verification

[johnscancella]

Currently we throw an exception at the first problem we discover when verifying a bag:

bagit-java/src/main/java/gov/loc/repository/bagit/verify/BagVerifier.java

Lines 151 to 173 in 0e4f80b

	@SuppressWarnings("PMD.AvoidInstantiatingObjectsInLoops")
	void checkHashes(final Manifest manifest) throws CorruptChecksumException, InterruptedException, VerificationException{
	final CountDownLatch latch = new CountDownLatch( manifest.getFileToChecksumMap().size());
	//TODO maybe return all of these at some point...
	final Collection<Exception> exceptions = Collections.synchronizedCollection(new ArrayList<>());
	for(final Entry<Path, String> entry : manifest.getFileToChecksumMap().entrySet()){
	executor.execute(new CheckManifestHashesTask(entry, manifest.getAlgorithm().getMessageDigestName(), latch, exceptions));
	}
	latch.await();
	if(!exceptions.isEmpty()){
	final Exception e = exceptions.iterator().next();
	if(e instanceof CorruptChecksumException){
	logger.debug(messages.getString("checksums_not_matching_error"), exceptions.size());
	throw (CorruptChecksumException)e;
	}
	throw new VerificationException(e);
	}
	}

Instead we should return a list of errors in the case that there is more than one problem with a bag and let the user of this library decide how to deal with them.