Minor followups for 4226

[carlaKC]

Addressing followups:

• Better error message for blinded trampoline forward.
• From u16 for error codes.

[ldk-reviews-bot]

👋 Thanks for assigning @valentinewallace 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.

[ldk-reviews-bot]

👋 The first review has been submitted!

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.32%. Comparing base (3e186bc) to head (2a53ffd).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/onion_payment.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4252   +/-   ##
=======================================
  Coverage   89.31%   89.32%           
=======================================
  Files         180      180           
  Lines      138966   138966           
  Branches   138966   138966           
=======================================
+ Hits       124121   124126    +5     
+ Misses      12218    12216    -2     
+ Partials     2627     2624    -3     

Flag	Coverage Δ
fuzzing	35.98% <0.00%> (+<0.01%)	⬆️
tests	88.68% <0.00%> (+<0.01%)	⬆️

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