Computer cmdlets should fail with error when not run via sudo on Unix

[SteveL-MSFT]

PR Summary

Computer cmdlets rely on shutdown command to work and doesn't have error handling when it fails so when run without root permissions it silently fails. Change is to check the exit code and emit the stderr message. Also changed to use CommandDiscovery to find shutdown instead of a hardcoded path.

PR Context

Fix #19802

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[daxian-dbw]

@SteveL-MSFT Did you take a look at #19780 and see if it tries to fix the same?

[SteveL-MSFT]

@daxian-dbw the two PRs are complimentary and addressing different issues

[ghost]

This pull request has been automatically marked as Review Needed because it has been there has not been any activity for 7 days.
Maintainer, please provide feedback and/or mark it as Waiting on Author

[JamesWTruher]

there's a potential collision between this and #19780

[pull-request-quantifier-deprecated]

This PR has 52 quantified lines of changes. In general, a change size of upto 200 lines is ideal for the best PR experience!

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Quantification details

Label      : Small
Size       : +42 -10
Percentile : 20.8%

Total files changed: 5

Change summary by file extension:
.cs : +27 -7
.resx : +3 -3
.ps1 : +12 -0

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[daxian-dbw]

LGTM