[racket] web server: response terminates servlet?

Hi all,

Using 6.0.1

I was under the (maybe mistaken) impression that calling response/...  
terminated the servlet thread.   It seems though that it doesn't.  I was 
hoping to use response as an early exit, so that my servlets look like: 
e.g.,

(define (user/exists request)
   (let* [
          (params   (request-bindings request))
          (username (exists-binding? 'username params))
          (response (make-hash))
         ]

     ; validate request
     (unless username
       (response/json (hasheq 'success #f 'msg "missing username" ))

     ; get request parameters
     (set! username (extract-binding/single 'username params))

     :

     ; send a real response
     (response/json response )
     ))


(define (response/json obj . cookies )
   (response/output
       (λ (op) (write-json obj op))
       #:code 200
       #:message #"OK"
       #:seconds (current-seconds)
       #:mime-type #"application/javascript"
       #:headers (map cookie->header cookies)
       ))

I expected that unless would bail out if the test fails. However, 
execution charges ahead into code that extracts post parameters and/or 
cookie values and then I get a contract error that terminates the 
servlet ... usually before my own error response is sent.

Is response supposed to terminate the servlet?  If not, can I wrap a 
let/ec around the whole servlet and do something like:

(define (user/exists request)
   (let/ec fail
      (let* [
             (params   (request-bindings request))
             (username (exists-binding? 'username params))
             (response (make-hash))
         ]

        ; validate request
        (unless username
          (response/json (hasheq 'success #f 'msg "missing username" )
           (fail))

        ; get request parameters
        (set! username (extract-binding/single 'username params))

        :

        ; send a real response
        (response/json response )
        )))

or do I have to arrange that the response always is the last value produced?

Thanks,
George