code: def add_error(self, path, error):
          
          self.errors = merge_errors(self.errors, self._make_error(path, error))
docs: Add error message for given field path.
  
          Example: ::
  
              builder = ValidationErrorBuilder()
              builder.add_error('foo.bar.baz', 'Some error')
              print builder.errors
              # => {'foo': {'bar': {'baz': 'Some error'}}}
  
          :param str path: '.'-separated list of field names
          :param str error: Error message