xkbcomp: improve a few log messages

Signed-off-by: Ran Benita <ran234@gmail.com>
scanner: don't fail over unknown escape sequence

This is too strict, and causes symbols/cz to fail parsing. Instead, just
emit a warning (not shown by default):
    xkbcommon: WARNING: cz:75:19: unknown escape sequence in string literal

https://bugs.freedesktop.org/show_bug.cgi?id=68056

Reported-By: Gatis Paeglis <gatis.paeglis@digia.com>
Signed-off-by: Ran Benita <ran234@gmail.com>