its: Improve error handling.
* gettext-tools/src/its.c: Include libxml/xmlerror.h.
(structured_error): New function.
(its_rule_list_add_from_file, its_rule_list_add_from_string,
its_rule_list_extract, its_merge_context_alloc): Exit upon fatal error.
Set the structured_error for the rest of the function.
(its_merge_context_merge, its_merge_context_write): Set the structured_error.
