feat(fetch): better errors messages for body-mixin methods

[KhafraDev]

Fixes #1653

The error messages are closer to chrome/firefox now. I don't think they need to be super descriptive.

edit: I considered adding the error header (ie. Response.text) but it would be relatively annoying to add the message to .json() as it just calls .text(), or the message would be completely wrong.

[codecov-commenter]

Codecov Report

Base: 94.89% // Head: 94.97% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (e4fe195) compared to base (dcd4e81).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1654      +/-   ##
==========================================
+ Coverage   94.89%   94.97%   +0.07%     
==========================================
  Files          53       39      -14     
  Lines        4803     2828    -1975     
==========================================
- Hits         4558     2686    -1872     
+ Misses        245      142     -103     

Impacted Files	Coverage Δ
index.js	80.00% <0.00%> (-20.00%)	⬇️
lib/handler/RedirectHandler.js	83.54% <0.00%> (-11.40%)	⬇️
lib/core/util.js	88.60% <0.00%> (-9.50%)	⬇️
lib/api/readable.js	82.75% <0.00%> (-8.63%)	⬇️
lib/core/request.js	92.54% <0.00%> (-7.46%)	⬇️
lib/proxy-agent.js	87.67% <0.00%> (-5.48%)	⬇️
lib/mock/mock-utils.js	95.13% <0.00%> (-4.87%)	⬇️
lib/core/connect.js	98.27% <0.00%> (-1.73%)	⬇️
lib/client.js	96.98% <0.00%> (-0.66%)	⬇️
lib/fetch/body.js
... and 13 more

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[mcollina]

lgtm