GUACAMOLE-1571: Properly wrap upload stream JSON errors.

guacamole/src/main/frontend/src/app/rest/services/tunnelService.js

@@ -316,7 +316,7 @@ angular.module('rest').factory('tunnelService', ['$injector',
 
             // Parse and reject with resulting JSON error
             else if (xhr.getResponseHeader('Content-Type') === 'application/json')
-                deferred.reject(angular.fromJson(xhr.responseText));
+                deferred.reject(new Error(angular.fromJson(xhr.responseText)));
 
             // Warn of lack of permission of a proxy rejects the upload
             else if (xhr.status >= 400 && xhr.status < 500)