Logstash: Add better user feedback when the HTTP API port is already in use

[ph]

If you start Logstash and the api http port is already in use it will throw this exception:

<Errno::EADDRINUSE: Address already in use - bind - Address already in use>

We should provide better feedback to the user, maybe a message similar to this one would help

Logstash tried to bind to port XXXX, but the port is already in use. You can specify a new port by launching logtash with the --http-port XXXX option.

[ceeeekay]

I think the error should also mention the API tried to bind to that port. If the user isn't aware the API exists, they will probably try to debug network inputs in their config.