Exception thrown when a you try register second default handler or handler under same name.

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

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

 String toString() => "Handler aready registed under name: $message";
}