Message5020

Actually it seems like we could do callable error handlers via nio's 
report error action. That would make the encoder/decoder return a 
CoderResult upon failure but without resetting its state

So we should be able to create a UnicodeError with its start/end/reason 
info from that CoderResult and the input Buffer (to pass to our error 
handler). Then we act upon the handler's result, restarting the 
encoder/decoder from where it left off if necessary