fix problem but not entirely

Error is passed to the div #scatterplot in shiny.  This error becomes the text in div #scatterplot.  Since we only remove the SVG and not the text, the error text was permanent.  Here is a way to handle this specifically, but I think for the "proper" way, I need to check into the shiny code for reactive error handling.

    //remove old graph    
    d3.select("#scatterplot").text(null);
    var svg = d3.select("#scatterplot").select("svg")
                        .remove();

Hope you enjoy maybe your first ever pull request.