Provide instruction if user does not add CORS

[invisal]

I would also see if you can specifically detect if a user doesn't launch with the CORS support enabled, as that will also be a common mistake.

I think the HTTP server will return a distinct error in that case.

[otoolep]

Pretty sure it does.

[invisal]

@otoolep

While, the console provides CORS error message, it is not possible for Javascript to catch that error. https://stackoverflow.com/a/19325710

Maybe when user fail to connect for some reason, we can provide list of can go wrong including the CORS instruction.

[otoolep]

OK, fair enough.