The $failedTestsCount was null

$Global:TestResults.FailedTestsCount was null.

I was trying to figure out why the ($p = Start-Process -FilePath
Powershell -ArgumentList "invoke-pester -enableExit") returned process,
wasn't reporting an error code above zero ($p.ExitCode == 0) when there
were pester errors.  I tracked it down to this line.
Merge pull request #44 from mitchelldavis/master

Exit-WithCode Error