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 would nominally respond that if we are passed like a div, then that div implicitly is the sandbox we're given, but my goals are just as well satisfied by having it throw an error and letting the user create an svg. ill amend.