make the stream deletion error message more specific

streams_map_outgoing_generic.go

@@ -153,7 +153,7 @@ func (m *outgoingItemsMap) DeleteStream(num protocol.StreamNum) error {
 
 	if _, ok := m.streams[num]; !ok {
 		return streamError{
-			message: "Tried to delete unknown stream %d",
+			message: "Tried to delete unknown outgoing stream %d",
 			nums:    []protocol.StreamNum{num},
 		}
 	}