fix for #2219, "provide a better error message when file= not found/accessible"

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

src/api.js

@@ -547,6 +547,10 @@ var WorkerTransport = (function WorkerTransportClosure() {
         this.workerReadyPromise.reject(data.exception.name, data.exception);
       }, this);
 
+      messageHandler.on('MissingPDF', function transportMissingPDF(data) {
+        this.workerReadyPromise.reject(data.exception.message, data.exception);
+      }, this);
+
       messageHandler.on('UnknownError', function transportUnknownError(data) {
         this.workerReadyPromise.reject(data.exception.message, data.exception);
       }, this);

src/util.js

@@ -175,6 +175,18 @@ var InvalidPDFException = (function InvalidPDFExceptionClosure() {
   return InvalidPDFException;
 })();
 
+var MissingPDFException = (function MissingPDFExceptionClosure() {
+  function MissingPDFException(msg) {
+    this.name = 'MissingPDFException';
+    this.message = msg;
+  }
+
+  MissingPDFException.prototype = new Error();
+  MissingPDFException.constructor = MissingPDFException;
+
+  return MissingPDFException;
+})();
+
 function bytesToString(bytes) {
   var str = '';
   var length = bytes.length;

src/worker.js

@@ -130,6 +130,12 @@ var WorkerMessageHandler = {
             exception: e
           });
 
+          return;
+        } else if (e instanceof MissingPDFException) {
+          handler.send('MissingPDF', {
+            exception: e
+          });
+
           return;
         } else {
           handler.send('UnknownError', {
@@ -185,8 +191,15 @@ var WorkerMessageHandler = {
             });
           },
           error: function getPDFError(e) {
-            handler.send('DocError', 'Unexpected server response of ' +
-                         e.target.status + '.');
+            if (e.target.status == 404) {
+              handler.send('MissingPDF', {
+                exception: new MissingPDFException(
+                  'Missing PDF \"' + source.url + '\".')});
+            } else {
+              handler.send('DocError', 'Unexpected server response (' +
+                            e.target.status + ') while retrieving PDF \"' +
+                            source.url + '\".');
+            }
           },
           headers: source.httpHeaders
         },