localed: be more verbose when keymap conversion to X11 fails

localed: be more verbose when keymap conversion to X11 fails

I was puzzled why "localectl set-keymap pl" does not change the X11 keymap.
Output a message at notice level, becuase not converting the X11 keymap
is most likely an error. We usually do not output non-debug messages
from "library" code, but this isn't really library code, it's split out
to a separate file only to allow it to be called from tests.

(pl is not converted because we only have a mapping for pl2. This is
intentional, even though we might want to change this. In any case, the
conversion code works correctly.)