better error

quad2/pipeline.rkt

@@ -39,10 +39,10 @@
                 (λ (ARG OTHER-ARG ...)
                   (when (current-use-preconditions?)
                     (unless (PRECOND-PROC ARG)
-                      (raise-argument-error 'PASS-NAME (format "~a" 'PRECOND-PROC) ARG)))
+                      (raise-argument-error 'PASS-NAME (format "~a (as precondition)" 'PRECOND-PROC) ARG)))
                   (define res (let () EXPRS ...))
                   (when (current-use-postconditions?)
                   (unless (POSTCOND-PROC res)
-                    (raise-argument-error 'PASS-NAME (format "~a" 'POSTCOND-PROC) res)))
+                    (raise-argument-error 'PASS-NAME (format "~a (as postcondition)" 'POSTCOND-PROC) res)))
                   res))
             'PASS-NAME))))]))