Be more polite to invalid DH-AES responses

I've learned that the AES decrypt function from pycrypto will throw a ValueError exception if the decrypt text is not a multiple of the AES block size, so we'll be more polite by catching the exception and telling the user that they failed to authenticate.

Also another commit that I forgot I did and got lost and somehow ended up on this branch that removes an unnecessary function.  It's not a function that did anything in the first place, either, so...