modules/policy.RPZ: log libzonefile errors as well

Fixes the first part of https://gitlab.labs.nic.cz/knot/knot-resolver/issues/453.

modules/policy/policy.lua

@@ -362,8 +362,17 @@ local function rpz_parse(action, path)
 		-- Policy triggers @NYI@
 	}
 	local parser = require('zonefile').new()
-	if not parser:open(path) then error(string.format('failed to parse "%s"', path)) end
-	while parser:parse() do
+	local ok, errstr = parser:open(path)
+	if not ok then
+		error(string.format('failed to parse "%s": %s', path, errstr or "unknown error"))
+	end
+	while true do
+		ok, errstr = parser:parse()
+		if errstr then
+			log('[poli] RPZ %s:%d: %s', path, tonumber(parser.line_counter), errstr)
+		end
+		if not ok then break end
+
 		local name = ffi.string(parser.r_owner, parser.r_owner_length)
 		local name_action = ffi.string(parser.r_data, parser.r_data_length)
 		rules[name] = action_map[name_action]