More exception handling options about graphiti-rails

[wagenet]

See also #5 (comment)

[richmolj]

Since we have access to the error, I wonder if the ExceptionHandler interface would best look like

def initialize(error, options)
  @error, @options = error, options
end

def status
  @options[:status] || 500
end

# etc

Then in AD we could

config = get_config_from_registry
handler = Graphiti::Rails::ExceptionHandler.new(e, config)
handler.status
handler.to_jsonapi # to_json, to_xml etc

# meaning we would do something like
render(handler.status, handler, content_type)

[wagenet]

For the first pass, I've decided to use the default handler and the InvalidRequest handlers only. I'm not initially encouraging people to register their own handlers, again this is because we may not even hit the GraphitiError handlers.