Tell the user if devcon.exe not found

[reedy]

If you run the file without devcon.exe actually existing, you get no output, and there's no clear indication of what is actually happening...

C:\Users\Administrator\Desktop>removedevices.js /verbose

C:\Users\Administrator\Desktop>

In made up pseudo code:
if not file exists("devcon.exe") {
echo "devcon.exe is missing...";
return;
}