Stop sending `channel_update` in onion failures

[tankyleo]

Per https://github.com/lightning/bolts/pull/1173.

Fixes #3277

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.88525% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.55%. Comparing base (c7627df) to head (9e30f25). Report is 267 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	82.97%	6 Missing and 2 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3345      +/-   ##
==========================================
- Coverage   89.67%   89.55%   -0.12%     
==========================================
  Files         126      127       +1     
  Lines      103165   103469     +304     
  Branches   103165   103469     +304     
==========================================
+ Hits        92510    92660     +150     
- Misses       7935     8104     +169     
+ Partials     2720     2705      -15     

Flag	Coverage Δ
	?

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[tankyleo]

I was reviewing this code during the Tokyo summit last week. Since you've already done the work, I added a couple of comments. Some might be unnecessary, and others are just to help me understand your changes 😄

Sorry I should have understood better, thank you for the review !