XEP-0198: Use different error message for bounces

When stanzas are bounced from the stream management queue (because the
session timed out or was closed for some other reason), use a different
error message so that this situation can be distinguished from other
cases.

src/ejabberd_c2s.erl

@@ -2886,7 +2886,7 @@ handle_unacked_stanzas(StateData)
 		    end;
 		false ->
 		    fun(From, To, El, _Time) ->
-			    Txt = <<"User session not found">>,
+			    Txt = <<"User session terminated">>,
 			    Err =
 				jlib:make_error_reply(
 				  El,
@@ -2898,7 +2898,7 @@ handle_unacked_stanzas(StateData)
 		?DEBUG("Dropping presence stanza from ~s",
 		       [jid:to_string(From)]);
 	   (From, To, #xmlel{name = <<"iq">>} = El, _Time) ->
-		Txt = <<"User session not found">>,
+		Txt = <<"User session terminated">>,
 		Err = jlib:make_error_reply(
 			El, ?ERRT_SERVICE_UNAVAILABLE(Lang, Txt)),
 		ejabberd_router:route(To, From, Err);