doc: include the error type in the request.resolve doc

[trusktr]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes I'm assuming documentation changes don't impact test results
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[trusktr]

I figure errors should be documented.

I went to look up the require.resolve documentation because I wanted to know what happens when the module is not found, and the doc didn't say anything about that. It only said that a string is returned, so my initial assumption was that maybe it returned an empty string in the case of a module not found.

[addaleax]

Landed in a220202