RGS Fails When Channel Was Removed

[TheBlueMatt]

If we remove a channel, for example because we got a failed payment with an error which told us to, we'll hit the following block, which shouldn't actually be an error, we should simply ignore this and move on. Fixing this is somewhat nontrivial as we still have to read the relevant data, we just have to ignore it, so it will require a bit of code motion.

                    .ok_or(LightningError {
                        err: "Couldn't find channel for update".to_owned(),
                        action: ErrorAction::IgnoreError,
                    })?;