core: Check STATUS_FAILURE in export operations.

* src/export.c (op_data_t): Add failure_code.
(export_status_handler): Set it.
(gpgme_op_export_ext, gpgme_op_export_keys): Also check the failure
code.

That is just in case we missed an error code. For example with older
gpg versions (e.g. 2.3.8) and exporting to a keyserver.