Alerts user when loading custom ontology fails! (#180)

closes #137 

* Alerts user when loading ontology fails!

* Indented and changed alert message.

* Simplified code by adding return statement.

* Removed extra parenthesis and added space!

* Added code to remove details section while loading custom.

js/tree-edam-stand-alone.js

@@ -109,6 +109,8 @@ function interactive_edam_browser(){
         reader.readAsText(file);
         reader.onload = function(event) {
             json = JSON.parse(event.target.result);
+            if(json == null)
+                return;
             if(typeof json.meta=="undefined"){
                 json.meta={"version":"v n/a"};
             }

js/tree-reusable-d3.js

@@ -811,13 +811,16 @@ function interactive_tree() {
         identifierToElement={};
         data_url = value;
         d3.json(value, function(json) {
+            if(json==null) {
+                alert('Unable to read content of "' + data_url + '"');
+            }else{
                 if(typeof json.meta=="undefined"){
                     json.meta={"version":"v n/a", "date":"n/a"};
                 }
                 json.meta.data_url=data_url;
                 chart.data(json);
             }
-        );
+        });
         return chart;
     };
     /**