Accept invalid IDs in useSlave and useInstance

Gracefully handle invalid IDs in useSlave and useInstance as this is
necessary when when the ID isn't known by the component using the hook,
since it can't conditionally call the hook.

packages/web_ui/src/model/instance.jsx

@@ -10,21 +10,20 @@ export function useInstance(id) {
 	let [instance, setInstance] = useState({ loading: true });
 
 	function updateInstance() {
-		if (!Number.isInteger(id)) {
-			setInstance({ missing: true });
-			return;
-		}
-
 		libLink.messages.getInstance.send(control, { id }).then(result => {
 			setInstance({ ...result, present: true });
 		}).catch(err => {
-			logger.log(`Failed to get instance: ${err}`);
+			logger.error(`Failed to get instance: ${err}`);
 			setInstance({ missing: true });
 		});
 
 	}
 
 	useEffect(() => {
+		if (!Number.isInteger(id)) {
+			setInstance({ missing: true });
+			return undefined;
+		}
 		updateInstance();
 
 		function updateHandler(newInstance) {

packages/web_ui/src/model/slave.jsx

@@ -9,11 +9,6 @@ export function useSlave(id) {
 	let [slave, setSlave] = useState({ loading: true });
 
 	function updateSlave() {
-		if (!Number.isInteger(id)) {
-			setSlave({ missing: true });
-			return;
-		}
-
 		// XXX optimize by requesting only the slave in question
 		libLink.messages.listSlaves.send(control).then(result => {
 			let match = result.list.find(i => i.id === id);
@@ -26,6 +21,10 @@ export function useSlave(id) {
 	}
 
 	useEffect(() => {
+		if (!Number.isInteger(id)) {
+			setSlave({ missing: true });
+			return undefined;
+		}
 		updateSlave();
 
 		function updateHandler(newSlave) {