systemctl: skip non-existent units in the 'cat' verb

When processing list of units (either provided manually or as a
wildcard), let's skip units for which we don't have an on-disk
counterpart, but note the -ENOENT error code and propagate it back to
the user.

Fixes: #14082
(cherry picked from commit a25457f5)
(cherry picked from commit 8b1bd174)