Don't start a dev instance if server port is already in use.

Currently, an Airflow development instance will start successfully even if the server_port (-p option) is not specified. The user will be instructed to start an SSH tunnel and redirect traffic using that port, but doing so will result in an unusable connection to the Airflow scheduler.

Check if server_port is free before starting a dev instance, and exit early if the port is already in use.