include ValidatorError message in ValidationError. fixes Automattic#2135.

Author: Ben Buckman

lib/document.js

@@ -1030,6 +1030,10 @@ Document.prototype.validate = function (cb) {
 Document.prototype.invalidate = function (path, err, val) {
   if (!this.$__.validationError) {
     this.$__.validationError = new ValidationError(this);
+
+    if (err instanceof ValidatorError) {
+      this.$__.validationError.message += ': ' + err.toString();
+    }
   }
 
   if (!err || 'string' === typeof err) {