SBRP GenerateScriptTests do not fail for test packages that have been removed from the repo

One of the packages the GenerateScriptTests validates against is System.Xml.ReaderWriter 4.0.11.  This package was removed from the repo because it was no longer being referenced yet the test isn't failing.

This is because the test validation logic is not validating the git diff error code correctly.  Alternatively it could also be validating that package exists in the source directory.