code: def error(self, error):
          
          # TODO: check length with value?
          # TODO: type checks (similar to value)
          if self.direction not in ['x', 'y', 'z'] and error is not None:
              raise ValueError("error only accepted for x, y, z dimensions")
  
          if isinstance(error, u.Quantity):
              error = error.to(self.unit).value
  
          self._error = error
docs: set the error