[FEATURE] Service##on_error private helper

The hepler adds error messages from argument to the service and
fails with Service::Invalid.

  on_error(messages)
  # => Hexx::Service::Invalid
[CHANGE] Hexx::Service::Message.from removed

The legacy method isn't used everywhere.
[CHANGE] Hexx::Service.allow_params individual setters and getters changed

Both the setters and getters provided by the .allow_params became aliases for corresponding params

  allow_params :name
  params["name"] # => nil

  name = "Hello!"
  name           # => "Hello!"
  params["name"] # => "Hello!"

  params["name"] = "Bye!"
  params["name"] # => "Bye!"
  name           # => "Bye!"

Earlier the setter didn't change params.

  name = "Hello!"
  name           # => "Hello!"
  params["name"] # => nil

  params["name"] = "Bye!"
  params["name"] # => "Bye!"
  name           # => "Hello!"

This was too confising sometimes.