Add assert messages for all assertions up until ol.renderer.vector.

src/ol/control/zoomtoextentcontrol.js

@@ -81,6 +81,6 @@ ol.control.ZoomToExtent.prototype.handleZoomToExtent_ = function() {
   var extent = goog.isNull(this.extent_) ?
       view.getProjection().getExtent() : this.extent_;
   var size = map.getSize();
-  goog.asserts.assert(goog.isDef(size));
+  goog.asserts.assert(goog.isDef(size), 'size should be defined');
   view.fitExtent(extent, size);
 };