Fix behavior if the PAC URL is not reachable

Steps to reproduce:

1. Prepare an environment with proxy server.
2. Create a PAC script that points to the proxy server. Publish the proxy URL in a web server (ex: http://127.0.0.1/proxy.pac)
3. Install and configure Drive to use the PAC script for proxy (http://127.0.0.1/proxy.pac)
4. Wait for Drive to complete sync.
5. Quit and re-launch Drive. Observe that Drive launches successfully without any issue.
6. Now shutdown the web server that hosts the PAC script.
7. Quit and re-launch Drive.
   Result: Drive launches and immediately quits.

Expected: Nuxeo Drive launches and if the server is not accessible (like pac script not available) then Drive tray icon should be in gray color.