Fix Web UI Browser when logged in user does not have READ granted on '/'

Step to reproduce:

• Create a user that has no READ granted for root '/' ( you can still grant permissions on a deeper part of the hierarchy)
• Login on Web UI with this user
• Open the navigation tree from the drawer menu
• See that you have error in the console about

  elements.html:41974 Resource request failed: Privilege 'Read' is not granted to 'jdoe'
  /nuxeo/ui/#!/home:1 Uncaught (in promise) Object {entity-type: "exception", code: "org.nuxeo.ecm.core.api.DocumentSecurityException", status: 403, message: "Privilege 'Read' is not granted to 'jdoe'"}
  

• Despite the error, you have the Root tree node available and clickable
• click on it
• see that the Web UI goes unstable