Tcontent Handler and notify errors to frontend

[cprmlao@hotmail.com]

I have a javascript code on frontend, where I call a content handler.  

When using TcontentHandlers, if the handler fail I need to inform the front end that an error occur.
The response of the handler is something as 

Code:

aReply.WriteString(here is the result);

But if there is an error, such as "400 error" or similar , how could I inform the frontend?
What could I put in writestring?

Luiz