core: Bless DBusError.SERVICE_UNKNOWN for D-Bus service problems on prepare()

As folks acquires more D-Bus-enabled backends, we need a common error code
for when the service can’t be started. In the Backend.prepare() documentation,
bless DBusError.SERVICE_UNKNOWN as this error code.

Also fix the BlueZ backend to emit it instead of a custom error code.

https://bugzilla.gnome.org/show_bug.cgi?id=711403
ofono: Throw a useful error if prepare() fails for a missing D-Bus service

Previously the error was completely unhelpful.

https://bugzilla.gnome.org/show_bug.cgi?id=711403