Ruby's Exception Error classes

You can create a subclass of an exception to make it more descriptive, but how should you set the default 'message'?

class MyError < StandardError
  # default message = "You've triggered a MyError"
end

begin
  raise MyError, "A custom message"
rescue Exception => e
  p e.message
end

begin
  raise MyError
raise Exception => e
  p e.message
end

The first should output 'A custom message'

The second should output 'You've triggered a MyError'

Any suggestions as to best practice?