its: Improve libxml error handling.

its: Improve libxml error handling.

When there is a non-fatal error, the previous code would just print a message
to stderr and continue the operation, without affecting the exit code.

* gettext-tools/src/its.c: Include <setjmp.h>, <stdarg.h>.
(xml_error_exit): New variable.
(structured_error): Longjmp to xml_error_exit.
(generic_error): New function.
(its_rule_list_add_from_file, its_rule_list_add_from_string,
its_rule_list_extract, its_merge_context_merge, its_merge_context_alloc,
its_merge_context_write): Invoke also xmlSetGenericErrorFunc. Use setjmp to
abort the operation when a non-fatal error occurs.