http_parser: fix error return in `Finish()`

http_parser_execute(..., nullptr, 0) returns either 0 or 1. The expectation is that no error must be returned if it is 0, and if it is 1 - a Error object must be returned back to user.

The introduction of llhttp and the refactor that happened during it accidentally removed the error-returning code. This commit reverts it back to its original state.

Fix: #24585 (closed)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines