Fail channel if we can't sign a new commitment tx during HTLC claim

Previously, we could fail to generate a new commitment transaction
but it simply indicated we had gone to doule-claim an HTLC. Now
that double-claims are returned instead as Ok(None), we should
handle the error case and fail the channel, as the only way to hit
the error case is if key derivation failed or the user refused to
sign the new commitment transaction.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -2709,15 +2709,12 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 					return Ok(())
 				},
 				Err(e) => {
-					// TODO: Do something with e?
-					// This should only occur if we are claiming an HTLC at the same time as the
-					// HTLC is being failed (eg because a block is being connected and this caused
-					// an HTLC to time out). This should, of course, only occur if the user is the
-					// one doing the claiming (as it being a part of a peer claim would imply we're
-					// about to lose funds) and only if the lock in claim_funds was dropped as a
-					// previous HTLC was failed (thus not for an MPP payment).
-					debug_assert!(false, "This shouldn't be reachable except in absurdly rare cases between monitor updates and HTLC timeouts: {:?}", e);
-					return Err(None)
+					let counterparty_node_id = chan.get().get_counterparty_node_id();
+					let (drop, res) = convert_chan_err!(self, e, channel_state.short_to_id, chan.get_mut(), &chan_id);
+					if drop {
+						chan.remove_entry();
+					}
+					return Err(Some((counterparty_node_id, res)));
 				},
 			}
 		} else { unreachable!(); }