src: add handle scope to `OnFatalError()`

For the report generation, we use `Environment::GetCurrent(isolate)`
which uses `isolate->GetCurrentContext()` under the hood, thus
allocates a handle. Without a `HandleScope`, this is invalid.

This might not strictly be allowed inside of `OnFatalError()`,
but it won’t make anything worse either.

PR-URL: https://github.com/nodejs/node/pull/25775


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>