The specification recommends using a length of 256 for onion errors, but
it doesn't say that we should reject errors that use a different length.

We may want to start creating errors with a bigger length than 256 if we
need to transmit more data to the sender. In order to prepare for this,
we keep creating 256-bytes onion errors, but allow receiving errors of
arbitrary length.

See the specification here: lightning/bolts#1021

Fixes #2438