Error message and missing <error/> child

Some of SPAM messages are sent as stanzas which are invalid if we compare them with XMPP specification, ie. <message/> stanza with type attribute set to error are sent without child element <error/> which is required for all packets of type error. This filter detects this kind of messages and marks them as SPAM.

This filter is identified by following id message-error-ensure-error-child.