Give HTLCHandlingFailed a reason of some kind

[TheBlueMatt]

With #2933 we're gonna expand substantially the cases where we generate an HTLCHandlingFailed. It would be nice to give the event a reason code for users to look at now.

[joostjager]

If I understand it correctly, HTLCHandlingFailed was already generated (without reason code), but just not in all failure cases. What is the background of adding the reason code? Is it nice to have as you say, or does it also unlock use cases that weren't possible previously because HTLCHandlingFailed wasn't generated always?

[carlaKC]

Discussed on LDK dev meeting today, motivation is to add more programatic observability to LDK - useful for tracking HTLC stats more granularly, for example. Generally just nice to have.