feat(webidl): add error messages

[KhafraDev]

Adds error messages throughout fetch (everything that was missing a message).

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My last PR for today. The WebIDL implementation should be done after this, unless there are any bugs.

[codecov-commenter]

Codecov Report

Merging #1508 (9456dc0) into main (98af06f) will decrease coverage by 0.00%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##             main    #1508      +/-   ##
==========================================
- Coverage   94.64%   94.63%   -0.01%     
==========================================
  Files          50       50              
  Lines        4575     4589      +14     
==========================================
+ Hits         4330     4343      +13     
- Misses        245      246       +1     

Impacted Files	Coverage Δ
lib/fetch/headers.js	94.40% <92.85%> (-0.56%)	⬇️
lib/fetch/webidl.js	98.28% <95.83%> (+0.09%)	⬆️
lib/fetch/body.js	98.52% <100.00%> (+0.01%)	⬆️
lib/fetch/formdata.js	96.07% <100.00%> (ø)
lib/fetch/request.js	82.23% <100.00%> (ø)
lib/fetch/response.js	94.25% <100.00%> (ø)

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[ronag]

The WebIDL implementation should be done after this, unless there are any bugs.

What about body.js?

[mcollina]

lgtm

[KhafraDev]

Nothing should need to be done in body.js; no methods take parameters and brand checks were recently added.