throw DecryptException on decrypt error (consistency fix)

Currently, switching my 4.1 install to 4.2 causes a _Warning: mcrypt_decrypt(): The IV parameter must be as long as the blocksize_ error

The error relates to decrypting cookie information, and there is a try/catch block that looks for a DecryptException to get around this, but an ErrorException is thrown when this error occurs.

This update forces the Encrypter to throw a DecryptException, which can then be caught and allow users to view the 4.2 based application without errors (and without cookies, so they'll have to log in again)