Friendlier warning if bluez is not installed on Linux

If you don't have `bluez` installed on Ubuntu, you'll get:

```
dbus_next.errors.DBusError: The name org.bluez was not provided by any .service files
```

It can simply be installed with `sudo apt-get install bluez` on Debian/Ubuntu. A friendlier error might be nice.

(Just noting for other wandering souls: On Ubuntu for Raspberry Pi, you should also install `pi-bluetooth`.)