Add friendly error if the TCP port is currently in use

[passbe]

If the port the framework is trying to run on is in use the error will be a stacktrace. Change this to provide a friendly error that explains what has happened and where in the configs to change the running port.

Google Code Issue: http://code.google.com/p/beef/issues/detail?id=520

[wadealcorn]

Fixed in:
76fdb64

Now outputs:
[21:53:57][!] Another process is already listening on port 3000.
[21:53:57][!] Is BeEF already running? Exiting...