Add an xerror_handler parameter to check_message and check_message_list.

Add an xerror_handler parameter to check_message and check_message_list.

* gettext-tools/src/msgl-check.h: Include xerror-handler.h.
(check_plural_eval, check_message, check_message_list): Add xerror_handler_ty
parameter.
* gettext-tools/src/msgl-check.c (check_plural_eval, check_plural, check_pair,
check_header_entry, check_message, check_message_list): Add xerror_handler_ty
parameter. Use it for error handling.
(struct formatstring_error_logger_locals): Add xeh field.
(formatstring_error_logger): Use it for error handling.
* gettext-tools/libgettextpo/gettext-po.c: Don't include po-xerror.h.
(po_file_check_all, po_message_check_all): Pass a stack-allocated xerror_handler
to check_message_list.
(po_message_check_format): Pass a stack-allocated xerror_handler to
check_message.
* gettext-tools/src/msgfmt.c (main, msgfmt_operand_list_add_from_directory): Use
the default xerror_handler.
* gettext-tools/src/msgmerge.c (match_domain): Likewise.