src: fix 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, which is being written in the file.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines