Add human-readable error messages corresponding to the recently added error codes, rework i18n stuff (intltool -> gettext), add French translation.

Signed-off-by: Lionel Debroux <lionel_debroux@yahoo.fr>
Remove dependency on C11 uchar.h, it's not supported widely enough yet. As a result, the code base becomes compilable in C99 mode: adjust build scripts.

Reported by wawachief and Adriweb.

Signed-off-by: Lionel Debroux <lionel_debroux@yahoo.fr>