[validate] Add a way to access the set of errors that occurred on validation

[mabner]

It would be nice if I could write a custom invalidHandler that iterated through the list of errors that occurred and then do whatever I like with them.

My particular use case was to create a custom popup box that had the list of errors in it, but there was no way to access those errors so I had to fall back to another solution.

[john]

Please file plugin bugs with the appropriate author on their bug tracker.