Have render() catch Reference errors by invoking the user callback

lib/carto/renderer.js

@@ -22,7 +22,11 @@ carto.Renderer.prototype.render = function render(m, callback) {
         effects: []
     });
 
-    tree.Reference.setVersion(this.options.mapnik_version);
+    try {
+      tree.Reference.setVersion(this.options.mapnik_version)
+    } catch (err) {
+      callback(err, null);
+    }
 
     var output = [];