Fixed flipper settings not being loaded / saved on fresh install

Summary:
I was running flipper-server on a fresh machine without Flipper installed and discovered that reading / writing settings failed since `~/.flipper` wasn't existing, due to using `access` instead of `pathExists`.

Added a warning about needing to restart the server after making changes, since that is tricky to do from the UI.

Fixed an issue in the settings screen, which would fs.stat as part of rendering, causing the Settings UI not to load.

Reviewed By: timur-valiev, aigoncharov

Differential Revision: D32984538

fbshipit-source-id: 2b2011ad9d84c72ac824d92a8c96f636237b8771

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

@@ -52,8 +52,9 @@ export function initializeRenderHost(
       );
     },
     restartFlipper() {
-      // TODO: restart server as well
-      window.location.reload();
+      window.flipperShowError!(
+        'Flipper settings have changed, please restart flipper server for the changes to take effect',
+      );
     },
     loadDefaultPlugins: getDefaultPluginsIndex,
     serverConfig: flipperServerConfig,