More exception handling options

[wagenet]

https://github.com/wagenet/graphiti-rails/blob/eeb6b6d91e2739a7d324205bba6975b52a668109/lib/graphiti/rails.rb#L54-L56

Right now when debugging exceptions we only use the default handler. That means there's no equivalent of GraphitiErrors.register_exception where a different handler can be specified. We also don't support the new InvalidRequest error.

[wagenet]

See also https://github.com/wagenet/graphiti-rails/issues/5#issuecomment-488733910

[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.