ipblocklist-functions.pl: Abort and return code if a list is empty or

ipblocklist-functions.pl: Abort and return code if a list is empty or
not parse-able.

In case the downloaded list is empty or the parser is not able to parse
it properly, the download_and_create_blocklist() function now exits and
will return "empty_list" as new error code.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>