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c:\test\xx>diff junk5.png junk6.png >nul & echo %errorlevel%
0
...
c:\test\xx>diff junk5.png junk4.png >nul & echo %errorlevel%
1

If you'd like to manually inspect system's failure, you can check all 
+possible failure modes by inspecting $? like this:
    if ($? == -1) {
...
    else {
        printf "child exited with value %d\n", $? >> 8;
    }