chore: Output failure telemetry if signing validation fails

[DaveTryon]

As called out in #501, telemetry is reporting success even if the manifest signing check fails. This PR fixes this via the following additions:

1. Add a new failure type so that we can appropriately distinguish this failure case from other failure cases
2. When a signing failure occurs, capture the failure in validFailures. This will be used in the finally block to capture the telemetry results
3. Extend the "this is an error" cases in LogResultsSummary and LogIndividualFileResults so that we don't generate unhelpful log output after signing validation fails

Tested locally by forcing the signing failure in the debugger and observing the rest of the execution. The resulting telemetry log reports the error as follows:

{
  "Result": "Failure",
  "Errors": {
    "Count": 1,
    "Errors": [
      {
        "Path": null,
        "ErrorType": "ManifestFileSigningError"
      }
    ]
  },
  // Other properties have been omitted for brevity
}

I left the Path field blank, since it wasn't available in the scope of this function and since it can be inferred from the Parameters object that exists in the raw file but that I omitted for brevity.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 59.70%. Comparing base (8609bce) to head (8684488).

Files	Patch %	Lines
...Api/Workflows/SBOMParserBasedValidationWorkflow.cs	0.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
- Coverage   59.70%   59.70%   -0.01%     
==========================================
  Files         248      248              
  Lines        7575     7576       +1     
  Branches      901      901              
==========================================
  Hits         4523     4523              
- Misses       2639     2640       +1     
  Partials      413      413              

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