Start fails when using remote-debugging-port cmd line switch

[kamesh-a]

@kevinsawicki thanks for working on this. It's awesome to directly test with electron app, Adding below lines to electron default app, is sufficient to reproduce this, thought of adding this as issue, in turn might help someone save time.

Ex : 
var app = require('app');
app.commandLine.appendSwitch('remote-debugging-port', '9222');

[kevinsawicki]

Looked into this a bit, not sure there is a way to externally detect this and show a better error message than a regular timeout.

[tinchoabbate]

I know this is already kind of documented here as an issue (a bit outdated in my defense 😃), but it would be really really nice if this was explicitly stated in the docs as a warning. I spent an entire day trying to figure out why on earth Spectron would not run any of my tests, until I ran into this issue to finally find the answer.