systemd.bbclass: Clarify error message

systemd.bbclass: Clarify error message

When this error is triggered, its a bit vague in specifying where the
issue is e.g.

ERROR: nbd-3.26.1-r0 do_package: nbd does not appear in package list, please add it

Some packages may intentionally remove PN from packages and find it
confusing as to why the system is still asking this to be in PACKAGES

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>