TokenProvider can fail if the server is offline

Summary: If the server is offline, the fetch request will fail and this was not properly handled.

Reviewed By: antonk52

Differential Revision: D50299455

fbshipit-source-id: dab8336dedaf93db049dc703a23f9e33935212be

desktop/flipper-ui-browser/src/index.tsx

@@ -42,9 +42,16 @@ async function start() {
       console.info(
         '[flipper-client][ui-browser] Get token from manifest instead',
       );
+      try {
         const manifestResponse = await fetch('manifest.json');
         const manifest = await manifestResponse.json();
         token = manifest.token;
+      } catch (e) {
+        console.warn(
+          '[flipper-client][ui-browser] Failed to get token from manifest. Error:',
+          e.message,
+        );
+      }
     }
 
     console.info(