Failed to bind should not panic

[rolznz]

If you run on a port that is already in use, the following will occur:

thread 'tokio-runtime-worker' panicked at src/lib.rs:513:45:
Failed to bind to listen address/port - is something else already listening on it?
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
signal: aborted (core dumped)

It would be nice instead to return an error message

[tnull]

This was fixed in #644.