Tests: update AJAX tests
Tests: update AJAX tests

* IE8 support for makeAjaxRequest test (xhr.response does not exist,
  responseText is supported by all major browsers)
* Update makeAjaxRequest test to confirm that the success callback is
  called and that the error callback is not
* Add a test for makeAjaxRequest with an invalid file and verify that
  error callback is called but the success callback is not

msalsbery/openseadragon,mcanthony/openseadragon,LarissaSmith/openseadragon,sul-dlss/openseadragon,janelia-flyem/openseadragon,daverome/openseadragon,jmnetus/openseadragon,lidasong2014/openseadragon,shaunstanislaus/openseadragon,openseadragon/openseadragon,mcanthony/openseadragon,ricochet2200/openseadragon,LafayetteCollegeLibraries/openseadragon,seanich/openseadragon,jmnetus/openseadragon,daverome/openseadragon,msalsbery/openseadragon,rdlester/openseadragon,goofwear/openseadragon,rdlester/openseadragon,Iced-Tea/openseadragon,avandecreme/openseadragon,seanich/openseadragon,shaunstanislaus/openseadragon,ricochet2200/openseadragon,janelia-flyem/openseadragon,avandecreme/openseadragon,lidasong2014/openseadragon,Iced-Tea/openseadragon,TMUND/openseadragon,goofwear/openseadragon,TMUND/openseadragon,openseadragon/openseadragon,LarissaSmith/openseadragon