Handle internally triggered Metalsmith.ware re-run

When using with `metalsmith-watch` out of the box, `metalsmith-serve` would crash with `EADDRINUSE`, as it gets executed a second time (`metalsmith-watch` just executes `Metalsmith.ware.run`).

This adds server tracking within the process, avoiding a second execution.
If server is up, terminate as early as possible
User friendly error for EADDRINUSE
Fix spacing