Handle bad server URL cases at account creation

Currently Drive is only considering web authentication as available if an HTTP connection to the Nuxeo Drive startup page <scheme>://<host>:<port>/<context>/drive_login.jsp sends a status code < 400 or == 401 (special case to handle anonymous user configuration), else it falls back on basic authentication.

This is sometimes not sufficient, for instance:

• Connection to http://localhost:8080 or http://localhost:8080/nuxeo2 results in a 404 (Not Found) thus a fall back on basic authentication even if drive_login.jsp actually exists in the server.
  => Maybe we could fetch the context path from the server and automatically add it to the server URL if not present / not right?
  We could also in this case add a button to retry web authentication with a new server URL.
• Connection to http://google.fr results in a 301 (Moved Permanently) thus opens the login window displaying the Google 401 page...
  => Maybe consider a 301 as web authentication not available?
• Connection to htttp://localhost:8080/nuxeo (extra 't' in the scheme) results in a 302 (Moved Temporarily, because of redirection) which is the default status in case of success thus opens the login window trying to load the URL as a static file and displaying a white page...
  => Maybe trap specific scheme errors?