[noise] recent changes about decryption failure

[Jason A. Donenfeld]

I just saw this:

+associated data. If a decryption error occurs the session has failed and the
+`CipherState` objects are deleted without sending or receiving further
+messages.

I'm not sure this is good behavior. I'm using Noise over UDP. If the
decryption fails, it means the packet wasn't legitimate and can be
dropped. But I don't want to kill the entire session because of this.
Otherwise it'd be a nasty DoS. Furthermore, packets are sometimes
corrupted on the wire. This means that the session is more or less
randomly deleted upon this corruption.