systemd.bbclass: improve error message when a service unit specified in...

systemd.bbclass: improve error message when a service unit specified in SYSTEMD_SERVICE is not found

The previous message was fairly useless without diving into the bbclass.

Signed-off-by: Chris Laplante <mostthingsweb@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>