Fix command failure not resulting in task failure

For whatever reason (I don't understand this stuff well) the
`win-podman-machine-main.ps1` script exits successfully despite the
final `Check-Exit` showing a non-zero exit code was detected.  Attempt
to fix this by throwing an exception instead of calling `Exit`.

Signed-off-by: Chris Evich <cevich@redhat.com>

contrib/cirrus/win-lib.ps1

@@ -51,7 +51,7 @@ function Check-Exit {
         # https://learn.microsoft.com/en-us/dotnet/api/system.management.automation.callstackframe
         $caller = (Get-PSCallStack)[1]
         Write-Host "Exit code = '$result' from $($caller.ScriptName):$($caller.ScriptLineNumber)"
-        Exit $result
+        Throw "Non-zero exit code"
     }
 }