Invalid SUBSCRIBE_UPDATE is a Protocol Violation (#479)

This case seems easily avoidable by a subscriber, so the simplest option
is to close the session.

Fixes #455
Fixes #467

draft-ietf-moq-transport.md

@@ -1079,7 +1079,9 @@ subscription can be made. The start Object MUST NOT decrease and when it increas
 there is no guarantee that a publisher will not have already sent Objects before
 the new start Object.  The end Object MUST NOT increase and when it decreases,
 there is no guarantee that a publisher will not have already sent Objects after
-the new end Object.
+the new end Object. A publisher SHOULD close the Session as a 'Protocol Violation'
+if the SUBSCRIBE_UPDATE violates either rule or if the subscriber specifies a
+Subscribe ID that does not exist within the Session.
 
 Unlike a new subscription, SUBSCRIBE_UPDATE can not cause an Object to be
 delivered multiple times.  Like SUBSCRIBE, EndGroup and EndObject MUST specify the