Allow htlc_accepted hook to respond with UPDATE failure messages.

[tnull]

Issue and Steps to Reproduce

I'm currently experimenting with a scenario in which I want to dynamically react to incoming HTLCs and, for example, adapt my channel policies. For this, I utilize the htlc_accepted hook offered by the plugin API and depending on the circumstances return a corresponding failure_message.

However, this currently does only work for failure messages that do not need additional parameters or require to append a channel update. That is, returning any failure message with the UPDATE/0x1000 failure code (cf. BOLT 04) currently results in an invalid fail onion.

It would be great if the plugin API would offer the ability to specify the additional fields required by the failure message or at least populate them with sane default values so failures with the UPDATE failure code could be returned by the htlc_accepted hook.

For my testing purposes, I implemented the latter approach, which can be seen here.

[cdecker]

Excellent idea, I'll see if I can add the full error payload as an argument.

[cdecker]

This was addressed in #3472