lnwire: allow longer failure messages

This fixes an incompatibility where lnd enforces a strict 256 byte
failure message, where as the spec sets this only as the recommended
length.

docs/release-notes/release-notes-0.16.0.md

@@ -5,6 +5,10 @@
 * Warning messages from peers are now recognized and
   [logged](https://github.com/lightningnetwork/lnd/pull/6546) by lnd.
 
+* Decrypt onion failure messages with a [length greater than 256
+  bytes](https://github.com/lightningnetwork/lnd/pull/6913). This moves LND
+  closer to being spec compliant.
+
 ## RPC
 
 * The `RegisterConfirmationsNtfn` call of the `chainnotifier` RPC sub-server