This commit adds a new field to the ForwardingError struct: ExtraMsg.
The purpose of this field is to allow the htlcswitch to tack on
additional error context to ForwardingError messages returned to the L3
router.
This commit renames the Deobfuscator interface to ErrorDecrypter and
the Obfuscator interface to ErrorEncrypter. With this rename, the
purpose of these two interfaces are a bit clearer.
Additionally, DecryptError (which was formerly Deobfuscate) now
directly returns an ForwardingError type instead of the
lnwire.FailureMessage.
This commit introduces a new type to the package: ForwardingError. It
wraps an existing lnwire.FailureMessage interface, and also includes
the _source_ of the error message. By including the source of the
message, the router can now prune the set of available routes down in
order to reduce the number of subsequent failures based on the source
of the error and the type of the error itself.
Within the network, it's important that when an HTLC forwarding failure
occurs, the recipient is notified in a timely manner in order to ensure
that errors are graceful and not unknown. For that reason with
accordance to BOLT №4 onion failure obfuscation have been added.
