properly handle fetch errors

priv/frontend/src/components/App.js

@@ -151,13 +151,23 @@ export default class App extends Component {
     log('loading metrics')
 
     try {
-      const resp = await fetch(`${DOMAIN}/api/status`)
-      const json = await resp.json()
-
-      this.setState({ metrics: json, loading: false })
+      var resp = await fetch(`${DOMAIN}/api/status`)
     } catch (err) {
-      this.setState({ error: err.toString() })
+      this.setState({
+        error: `Network error: ${err}`,
+      })
     }
+
+    if (!resp.ok) {
+      this.setState({
+        error: `Failed to fetch stats (${resp.status} ${resp.statusText})`,
+      })
+      return
+    }
+
+    const json = await resp.json()
+
+    this.setState({ metrics: json, loading: false })
   }
 
   render () {