Fixes bug where get_entity_count never returned an API error..

script/fetcher.lua

@@ -17,7 +17,9 @@ this.overlays.get_entity_count = function(parent, entity)
     return parent.get_entity_count(entity)
   end)
 
-  return data
+  if res then return data end
+
+  return nil
 
 end
 
@@ -45,9 +47,13 @@ this.callOverlayFunction = function(functionName, callback, name, parent, args)
 
     --data = parent.get_entity_count(args[1])
     data = this.overlays.get_entity_count(parent, args[1])
-    this.doCallback(callback, name, data)
 
-    return true
+    if data == nil then
+      return false
+    else
+      this.doCallback(callback, name, data)
+      return true
+    end
 
   -- flow statistics
   elseif functionName == "get_flow_count" and parent.object_name == "LuaFlowStatistics" then