lslogins: return 1 on "lslogins nonexisting"

lslogins: return 1 on "lslogins nonexisting"

The default behavior for -l and -g is to silently ignore unknown login
names, but this is very confusing when you explicitly specify just one
login name.

Note that the current implementation also prints empty "Last log" for
nonexisting user. It seems ugly.

 # lslogins nonexisting

 Last logs:

new version:

 # lslogins nonexisting
 lt-lslogins: cannot found 'nonexisting'
 # echo $?
 1

The -l and -g behaviour has not been changed.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1614967
Signed-off-by: Karel Zak <kzak@redhat.com>