Display the Login page instead of empty dialog

During development it is common to erase the local development datastore
and start over. However if the browser in use still had a valid
authentication cookie (very common), a blank dialog would be displayed
because the cookie resolved to a non-existent account. This change
results instead with either the login page, or Google authentication
being invoked (depending on configuration). Note: This code should *not*
get triggered in production.

Change-Id: Iafbc7c774c392d99073342c49108dc6774e715a0

appinventor/appengine/src/com/google/appinventor/server/OdeAuthFilter.java

@@ -145,7 +145,16 @@ public class OdeAuthFilter implements Filter {
     // find the UserData object.
     String lemail = localUser.getUserEmail();
     if (lemail.equals("")) {
-      response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
+      // We send a SC_PRECONDITION_FAILED which will cause the login page to
+      // be displayed (or the use of Google Authentication if that is the only
+      // mechanism enabled). This should *not* happen in production. However
+      // it happens all the time in development when people do an "ant clean"
+      // followed by an "ant". This results in their development datastore being
+      // erased. But their browser still contains a valid authentication cookie,
+      // but the userId no longer exists. It is then automatically created
+      // in code called before here, but the e-mail address is not set. So
+      // we error out here.
+      response.setStatus(HttpServletResponse.SC_PRECONDITION_FAILED);
       return;
     }