more verbose unknown node error message in code generator

lib/6to5/generation/generator.js

@@ -115,6 +115,7 @@ CodeGenerator.prototype.print = function (node, parent, opts) {
   if (!node) return "";
 
   var self = this;
+
   opts = opts || {};
 
   var newline = function (leading) {
@@ -170,7 +171,7 @@ CodeGenerator.prototype.print = function (node, parent, opts) {
 
     this.printTrailingComments(node, parent);
   } else {
-    throw new ReferenceError("unknown node " + node.type);
+    throw new ReferenceError("unknown node of type " + JSON.stringify(node.type) + " with constructor " + JSON.stringify(node && node.constructor.name));
   }
 };