Follow ups to "Remove convert channel err macro"

[joostjager]

Follows up to #4275

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 99.39024% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.39%. Comparing base (593d6aa) to head (867bd6e).

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	99.39%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4281      +/-   ##
==========================================
+ Coverage   89.37%   89.39%   +0.02%     
==========================================
  Files         180      180              
  Lines      139847   139846       -1     
  Branches   139847   139846       -1     
==========================================
+ Hits       124983   125012      +29     
+ Misses      12269    12253      -16     
+ Partials     2595     2581      -14     

Flag	Coverage Δ
fuzzing	35.22% <82.31%> (+<0.01%)	⬆️
tests	88.72% <99.39%> (+0.02%)	⬆️

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