External Browser: Try a default port for socket.io first

Try to use a hard-coded port, 5678, and only ask
for a random free port when it is not available.
Users that have only one connection thus get by
most of the time without having to change the port
in their HTML whenever LT restarts.

I think it is also better to log errors than to
swallow them.
I am, of course, open to discussions.