Serialize errors before invoking postMessage

Serialize errors to make sure that the callback is still invoked when
an error is thrown.

Firefox:
"DataCloneError: The object could not be cloned."

Chrome:
"DataCloneError: Failed to execute 'postMessage' on 'WorkerGlobalScope': An object could not be cloned."

src/shared/util.js

@@ -1504,6 +1504,10 @@ function MessageHandler(name, comObj) {
             data: result
           });
         }, function (reason) {
+          if (reason instanceof Error) {
+            // Serialize error to avoid "DataCloneError"
+            reason = reason + '';
+          }
           comObj.postMessage({
             isReply: true,
             callbackId: data.callbackId,