Detect premature worker load error

Rejects the ready promise when the worker fails to load, e.g. due to
network errors.

src/display/api.js

@@ -1302,11 +1302,25 @@ var PDFWorker = (function PDFWorkerClosure() {
           // https://bugzilla.mozilla.org/show_bug.cgi?id=683280
           var worker = new Worker(workerSrc);
           var messageHandler = new MessageHandler('main', 'worker', worker);
+          var terminateEarly = function() {
+            this._readyCapability.reject(new Error('Worker was destroyed'));
+            messageHandler.destroy();
+            worker.removeEventListener('error', onWorkerError);
+            worker.terminate();
+          }.bind(this);
+
+          var onWorkerError = function(event) {
+            if (!event.message && !this.destroyed && !this._webWorker) {
+              // Worker failed to initialize due to a load error.
+              terminateEarly();
+            }
+          }.bind(this);
+          worker.addEventListener('error', onWorkerError);
+
           messageHandler.on('test', function PDFWorker_test(data) {
+            worker.removeEventListener('error', onWorkerError);
             if (this.destroyed) {
-              this._readyCapability.reject(new Error('Worker was destroyed'));
-              messageHandler.destroy();
-              worker.terminate();
+              terminateEarly();
               return; // worker was destroyed
             }
             var supportTypedArray = data && data.supportTypedArray;
@@ -1333,10 +1347,9 @@ var PDFWorker = (function PDFWorkerClosure() {
           });
 
           messageHandler.on('ready', function (data) {
+            worker.removeEventListener('error', onWorkerError);
             if (this.destroyed) {
-              this._readyCapability.reject(new Error('Worker was destroyed'));
-              messageHandler.destroy();
-              worker.terminate();
+              terminateEarly();
               return; // worker was destroyed
             }
             try {