Better error messages for wasm network errors

[ianthomas23]

This PR uses better error messages for wasm network errors. These error messages occur on the JavaScript side and are passed back to the C++ for display on stderr. Unfortunately some of the error messages are not very informative for end users, such as if there isn't a CORS proxy:

js-shell: git clone https://github.com/ianthomas23/cockle-playground
Cloning into 'cockle-playground'...
error: NetworkError: Failed to execute 'send' on 'XMLHttpRequest': Failed to load 'https://github.com/ianthomas23/cockle-playground/info/refs?service=git-upload-pack'.

This error is now displayed as:

js-shell: git clone https://github.com/ianthomas23/cockle-playground
Cloning into 'cockle-playground'...
error: network error sending request to https://github.com/ianthomas23/cockle-playground/info/refs?service=git-upload-pack, see the browser console and/or network tab for more details

Unfortunately we cannot be more specific as the real reason for a failed XMLHttpRequest.send is browser-specific and is not available via a standard browser JavaScript API. Some browsers report something useful in the browser console, others don't, and usually there is something useful but necessarily technical in the browser dev tools network tab.

Longer term we can probably do better than this, but for now it is at least an improvement on the current behaviour.

I have tested this manually on chrome, firefox and safari.

Closes #130.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.22%. Comparing base (85b406f) to head (b1df300).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
- Coverage   88.27%   88.22%   -0.06%     
==========================================
  Files          61       61              
  Lines        2730     3065     +335     
  Branches      330      330              
==========================================
+ Hits         2410     2704     +294     
- Misses        320      361      +41     

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[ianthomas23]

WebAssembly tests passing: https://github.com/QuantStack/git2cpp/actions/runs/24778479938

Codecov fails as we don't have a way of measuring code coverage in the wasm build.