Added warning to meta.js

gui/slick/js/new/meta.js

 function metaToBool(pyVar){
     var meta = $('meta[data-var="' + pyVar + '"]').data('content');
+    if(meta === undefined){
+        console.log(pyVar + ' is empty, did you forget to add this to main.mako?');
+        return meta;
+    } else {
         meta = (isNaN(meta) ? meta.toLowerCase() : meta.toString());
         return !(meta === 'false' || meta === 'none' || meta === '0');
     }
+}
 
 function getMeta(pyVar){
     return $('meta[data-var="' + pyVar + '"]').data('content');