chore: better error message for FST_REQ_FILE_TOO_LARGE

[erfanium]

I think the please check multipart config part is extra to be in response body.
that's a 4xx error, so we should assume the client is wrong.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[erfanium]

@Uzlopak can you re-run the workflow? i'm sure something is wrong with ci

[mcollina]

lgtm

[is2ei]

I re-run the workflow and it still fails.
I investigated why it fails.
It seems lockfile version 3 causes some issues on Windows + Node.js v14.
tapjs/tapjs#843 (comment)

node-tap has changed the lockfile version in v16.3.5.
tapjs/tapjs@v16.3.4...v16.3.5

I created minimum example:
https://github.com/is2ei/_tap_16_0_0_windows/actions