Update error message for authenticode check

[alerickson]

PR Summary

Adds a more descriptive error message for when authenticode checks fail:

PR Context

Resolves issue #1698

PR Checklist

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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Breaking changes
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  • Documentation needed
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• User-facing changes
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  • Documentation needed
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• Testing - New and feature
  • N/A or can only be tested interactively
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[anamnavi]

LGTM