Don't call put-message without unwinding the stack

When handling exceptions, as this is error prone.

knots/resource-pool.scm

@@ -650,12 +650,27 @@ available.  Return the resource once PROC has returned."
         (lambda ()
           (with-exception-handler
               (lambda (exn)
-                (print-backtrace-and-exception/knots exn)
+                ;; Unwind the stack before calling put-message, as
+                ;; this avoids inconsistent behaviour with
+                ;; continuation barriers
                 (put-message (resource-pool-channel pool)
                              `(return ,resource))
                 (raise-exception exn))
             (lambda ()
-              (proc resource))))
+              (with-exception-handler
+                  (lambda (exn)
+                    (match (fluid-ref %stacks)
+                      ((stack-tag . prompt-tag)
+                       (let ((stack (make-stack #t
+                                                0 prompt-tag
+                                                0 (and prompt-tag 1))))
+                         (raise-exception
+                          (make-exception
+                           exn
+                           (make-knots-exception stack)))))))
+                (lambda ()
+                  (proc resource))))
+            #:unwind? #t))
       (lambda vals
         (put-message (resource-pool-channel pool)
                      `(return ,resource))