add sanity check to warn if extension url isn't properly set up
Created by: tttp
It seems it's a common pattern that the extension url isn't properly configured, so we don't load properly the d3 and dc and pals
It leads to a dataviz that is mostly empty, with no graph. Instead of expecting the users to check if there is a js error ("dc not an object" type), let's test it and display a proper error message
X+