src: error reporting on CPUUsage

Currently we are returning the stringified error code while in other
process methods we are throwin a UVException and only exclusion is in
the CPUUsage. Converted it to follow the convention.

PR-URL: https://github.com/nodejs/node/pull/34762


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Mary Marchini <oss@mmarchini.me>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>