Throw if npm install returns non-zero exit code

[guillermooo]

Closes #251

PS> invoke-DscResource -ModuleName NpmDsc -Name NpmPackage -Method set -Property @{ Name = 'mariozechner/pi-coding-agent'; Global = $true }
Exception: Command 'npm install mariozechner/pi-coding-agent -g' failed: error code 128; error An unknown git error occurred; error command git --no-replace-objects ls-remote ssh://git@github.com/mariozechner/pi-coding-agent.git; error git@github.com: Permission denied (publickey).; error fatal: Could not read from remote
repository.; error Please make sure you have the correct access rights; error and the repository exists.; error A complete log of this run can be found in: C:\Users\xxx\AppData\Local\npm-cache\_logs\2026-03-13T20_02_00_172Z-debug-0.log

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Microsoft Reviewers: Open in CodeFlow

[denelon]

@guillermooo can you update the branch for this PR?

[guillermooo]

@denelon Done!

[denelon]

Thanks! I'll ask someone to review the changes.

[AmelBawa-msft]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[AmelBawa-msft]

Test failure is unrelated to this change