summary: create a new Exception class for missing PDF's, use it in place of generic add new MissingPDFException to util.js handle MissingPDF in api.js handle MissingPDF in viewer.js, using new missing_file_error message add new missing_file_error to l10n/en-US/viewer.properties send MissingPDF from WorkerMessageHandler's loadDocument send MissingPDF from GetDocRequest handler