Clear error messages for renderTo

Make Plottable easier to use by giving clear errors when the `renderTo` contract is not satisfied.

Right now if you `renderTo("invalid_selector_lol")` it gives an obscure error message, and if you accidentally put in a div or something it will similarly blow up. Now it checks the arguments and gives clear feedback on what went wrong.
Based on discussion with @themadcreator 
I'm not sure we want to auto-create a node; In general our principle has been to not touch the DOM outside of a sandbox we've been given.