Improve routee-powertrain model error message

[nreinicke]

Right now if we pass in a model_name for computing energy routes and the SpeedGradeModel can't find a model that matches that name we just get a simple error:

No energy model found with name flying_car"

I would be helpful for the user if we could list out all the available energy models so they can pick one from the error message rather than having to dig into the config (especially if this query was executed on a opaque server).