Fix Error handling in layershell producer (#61)

previous error handling resulted in a softlock when the connection
to the compositor was lost

src/backend/producer/dummy.rs

@@ -24,9 +24,13 @@ impl Default for DummyProducer {
 }
 
 impl EventProducer for DummyProducer {
-    fn notify(&mut self, _: ClientEvent) {}
+    fn notify(&mut self, _event: ClientEvent) -> io::Result<()> {
+        Ok(())
+    }
 
-    fn release(&mut self) {}
+    fn release(&mut self) -> io::Result<()> {
+        Ok(())
+    }
 }
 
 impl Stream for DummyProducer {