raise a distinct LoggedOutError when doing a forced-logged-out

refs CNVS-14595

instead of waiting for an InvalidAuthenticityToken to catch it

test plan:
 * log in to one browser, and go to an assignment page
 * in another browser (or incognito) log in and log out as the same user
 * in browser 1, refresh. you should have to login again
 * watch your requests - pings should be happen periodically, and
   succeeding
 * log in/out in browser 2
 * back in browser 1, a ping should fail. check your logs or
   errorreports - it should be a LoggedOutError, and InvalidAccessToken
 * refresh and log back in
 * log in/out in browser 2
 * try to edit a rubric (or another AJAX request that's not API) -
   it should fail, again *not* with InvalidAccessToken

Change-Id: I04c72e12fbcee7dd0aa4ce7dafcb698167a82015
Reviewed-on: https://gerrit.instructure.com/38755
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>