Replace oSettings.oLanguage.sProcessing with error message

I am trying to replace the Processing.. text with the server side error message when an 500 error occurs .. the below snippet doesn't work, any pointers ?

fnServerData: function(sUrl, aoData, fnCallback, oSettings) {

....
error: function (jqXHR, textStatus, errorThrown) {

oSettings.oLanguage.sProcessing = "Server error :" + errorThrown

...
}
}