Generate `HTLCHandlingFailed` when we don't forward

[TheBlueMatt]

Currently, if we receive an HTLC and decide that we aren't gonna forward it at all, we never generate the HTLCHandlingFailed event, but we should. Followup on #1392

[dunxen]

I believe the only case where this is not covered is when the short channel ID to forward on is unknown in process_pending_htlc_forwards. Or should I be looking elsewhere too?

[TheBlueMatt]

I know its at least also not generated if the peer is currently disconnected.

[dunxen]

I know its at least also not generated if the peer is currently disconnected.

Cool. I'll dig a bit deeper.