Downgrade "Unexpected onionskin length after decryption" warning

It's now a protocol-warn, since there's nothing relay operators can
do about a client that sends them a malformed create cell.

Resolves bug 12996; bugfix on 0.0.6rc1.
Merge remote-tracking branch 'arma/bug12996b' into maint-0.2.5
Merge remote-tracking branch 'origin/maint-0.2.5'