feat(http/unstable): HttpError

[iuioiua]

This PR brings back the HttpError class, which I regret removing in #3736 (sorry Kitson 🥲). It's a very versatile class that can be called anywhere in the call stack and handled in a unified way in the "master" request handler. I've been using this in my own projects and love it.

This class is already implemented in Oak (not identially) and Fresh (a more minimal version).

I think it'd also be worth updating handlers within @std/http to throw errors instead of returning responses, allowing the dev to handle their own errors.

Written by me. Checked by Claude Code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (a496da2) to head (5ce5c24).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7132   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files         634      635    +1     
  Lines       51801    51816   +15     
  Branches     9329     9330    +1     
=======================================
+ Hits        49011    49026   +15     
  Misses       2216     2216           
  Partials      574      574           

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