[rush] Fix logging for non-zero exit codes

[dmichon-msft]

Summary

Fixes #4882

Child process that only writes to stdout but is configured to exit with code 2.
Before:

After:

Details

With this change, Rush internally writes any fatal error from an Operation to the stderr stream of the StdioSummarizer instead of the stdout stream. This effectively makes the error seen by Rush be treated as lower priority for the logs than any stderr output from the Operation itself.

How it was tested

Added process.exitCode = 2 to the end of the build script in api-extractor-lib1-test.

Impacted documentation

Anything about the error reporting for non-zero exit codes.