Handle a non-200 status code as an error

src/js/hooks/useWebService.ts

@@ -8,9 +8,19 @@ export function useWebService(): [Maybe<TResponse>, Maybe<Error>] {
   const [response, setResponse] = useState<Maybe<TResponse>>(null);
   const [error, setError] = useState<Maybe<Error>>(null);
 
+  function receive(res: Response) {
+    if (res.status === 200) {
+      return res.json();
+    } else {
+      const message = "There was an unexpected response from the web service. " +
+                      `The status code was ${res.status}`;
+      throw new Error(message);
+    }
+  }
+
   useEffect(() => {
     fetch(endpoint)
-      .then((res) => res.json())
+      .then(receive)
       .then((json) => setResponse(Response(json)))
       .catch((err) => setError(err));
   });