(EILSEQ, ENOTSUP): New macros, if not defined by errno.h.

(<error.h>): Do not include.
(unicode_to_mb): Return int, not void.  New arg for failure callback.
Invoke the failure callback instead of invoking 'error'.
(print_unicode_success): Renamed from fprintf_callback.  Now extern.
Return -1 on failure, 0 on success.
(print_unicode_failure): New function.
(print_unicode_char): Return int.  Adjust to changes to unicode_to_mb.
Do not report error: instead, print an approximation.