Tested on Canary v77.0.3817.0
Don't forget to enable portals (chrome://flags/#enable-portals)
When the <portal> loads a normal page we will get one onload event but when the page doesn't exist or an error is thrown we will get multiple onload events
This POC depend on timing and it may give false postivies
Testing ports: {80,55,69,77,88,1337,99}
Updated 7-June-2019 (better timing)