systemctl: print a warning when trying to import a nonexistent variable

I was quite confused what is happening:
$ XXX=xxx
$ systemctl --user import-environment XXX
$ systemctl --user show-environment | grep XXX
(nothing)

Obviously, 'export XXX' was missing. Without any indication why the
export is not happening, this can be hard to figure out.

Another option would be to error out. But so far we didn't, and doing
that could break some script which optimistically tries to export some
variables, if present.