Exception thrown when a MultiRequestHandler can pass one of multiple request to any handler

class UnknownHandlerException implements Exception {
 /**
  * A message describing the format error.
  */
 final String message;

 /**
  * Creates a new FormatException with an optional error [message].
  */
 const UnknownHandlerException([this.message = ""]);

 String toString() => "UnknownHandlerException: $message";
}