refactor(http): inline serveFallback() util

[kt3k]

This PR inlines serveFallback util in file_server.

This PR also removes the handling of URIError. This error can't happen now because url is taken from request.url which should be always a valid url.

This prepares for the implementation of #5875

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.25%. Comparing base (fb79076) to head (523fa8d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
http/file_server.ts	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5917      +/-   ##
==========================================
+ Coverage   96.24%   96.25%   +0.01%     
==========================================
  Files         491      491              
  Lines       39443    39436       -7     
  Branches     5811     5810       -1     
==========================================
- Hits        37961    37959       -2     
+ Misses       1438     1433       -5     
  Partials       44       44              

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