Add instructions to port occupied error for using lsof

[rglover]

This can happen in weird instances where cleanup of the app doesn't occur. When it does, the error should give steps for clearing the port, e.g.:

Port 2600 is already occupied. To start Joystick on this port, clear it and try again.

Steps to clear (in your terminal):

1. Run lsof -i :2600 in your terminal.
2. Copy the process ID for the node process running on that port.
3. Run kill <processId> in your terminal.

Repeat the above process for port 2601 to clear Joystick's HMR server.