http: 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
PR-URL: https://github.com/nodejs/node/pull/24738


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>