raising ArgumentError if exposure_options[:using] does not respond to represent

The current NoMethodError that gets raised requires time to trace and debug. The ArgumentError that is raised now gives a little more information and context to fix the issue quicker.
Just re-read this. It doesn't have to be a class that inherits `Grape::Entity`, that is just one possible class. I think it should say something like "#{exposure_options[:using].class} must implement represent.", what do you think?