[Fix]: Improve test results failure handling and reporting

[MariusStorhaug]

Problem

The Get-PesterTestResults action needs better error handling and reporting when processing test results. Currently, it doesn't properly handle cases where test result properties might be null or missing, and error messages aren't granular enough to diagnose specific failure reasons.

Proposed Solution

Enhance the test results processing logic to:

• Add null-checking before accessing test result properties (Tests, Passed, Failed, Inconclusive, NotRun)
• Provide more granular failure reason reporting for each type of test failure
• Track missing result files separately from unexecuted tests
• Generate specific error messages for each failure condition

Expected Outcome

Users will receive clearer, more actionable error messages when tests fail, making it easier to diagnose and fix issues with their test suites.