docs: def error(self, code=500, callback=None):
          """ Register an output handler for a HTTP error code. Can
              be used as a decorator or called directly ::
  
                  def error_handler_500(error):
                      return 'error_handler_500'
  
                  app.error(code=500, callback=error_handler_500)
  
                  @app.error(404)
                  def error_handler_404(error):
                      return 'error_handler_404'
  
          """
  
          def decorator(callback):
              if isinstance(callback, basestring): callback = load(callback)
              self.error_handler[int(code)] = callback
              return callback
  
          return decorator(callback) if callback else decorator