dragging & dropping a folder into the sketch window throws exception #441
Description
Created by: benfry
Normally, dragging and dropping a file onto the sketch window will add it to the data folder (if a data file) or add it as a tab (if it's source code, or at least editable by the current Mode).
Dropping a folder into the window throws a confusing exception; just need to detect this and show a better warning/error message. Could also recursively add to the data folder, but probably better to have users do that manually, since it's 1) extra code and 2) maybe not the expected behavior.