DB schema: Fix IDNA Unicode decode in conversion script

* The idna library used leaves the Unicode codepoint validation
  to the Unicode implementation and does not catch the exceptions
  from therein. So, catch these explicitly.

db/IDNA_convert.sql

@@ -8,7 +8,7 @@ CREATE FUNCTION idnadecode(domain text) RETURNS text AS $$
 	import idna
 	try:
 		return idna.decode(domain)
-	except idna.core.IDNAError:
+	except (idna.core.IDNAError, UnicodeError):
 		return domain
 $$ LANGUAGE plpython3u;