systemctl: show error when help for unknown unit is requested

Fixes #20189. We would only log at debug level and return failure, which looks
like a noop for the user.

('help' accepts multiple arguments and will show multiple concatenated man
pages in that case. Actually, it will also show multiple concatenated man pages
if the Documentation= setting lists multiple pages. I don't think it's very
terribly useful, but, meh, I don't think we can do much better. If a user
requests a help for a two services, one known and one unknown, there'll now be
a line in the output. It's not very user friendly, but not exactly wrong too.)