Fix: Only report a general Ajax error when request is complete

- Previously if you cancelled an Ajax request, or the browser did (for
  example following a link) the error handler would be entered and show
  an alert. This just ensures that the request is complete before
  showing an error
Merge branch 'master' of github.com:DataTables/DataTablesSrc
Fix DataTables/TableTools #46 - fnGetData should return null if row not
found

- The old fnGetData method would return null, rather than undefined for
  some reason if a given row was not found in the DataTable. The new
  wrapper api for the legacy interface should do the same.