fixed no error was raised if Casper.viewport() was called before casper having been started

@@ -881,6 +881,9 @@
          * @return Casper
          */
         viewport: function(width, height) {
+            if (!this.started) {
+                throw new Error("Casper must be started in order to set viewport at runtime");
+            }
             if (!isType(width, "number") || !isType(height, "number") || width <= 0 || height <= 0) {
                 throw new Error("Invalid viewport width/height set: " + width + 'x' + height);
             }