Update render.rkt

rely on default exception messages, which are informative

pollen/render.rkt

@@ -234,19 +234,18 @@
 (define ram-cache (make-hash))
 
 (define (get-external-render-proc v)
-  (cond
-    [(and (list? v)
-          (eq? 2 (length v))
-          (module-path? (first v))
-          (symbol? (second v)))
-     (let ([mod (first v)]
-           [render-proc-id (second v)])
+  (match v
+    [(list (? module-path? mod) (? symbol? render-proc-id))
      (with-handlers ([exn:fail:filesystem:missing-module?
-                      (lambda (e) (error 'external-renderer "cannot open module ~a" mod))])
-       (dynamic-require mod
-                        render-proc-id
-                        (lambda () (error 'external-renderer "~a is not provided by ~a" render-proc-id mod)))))]
-    [else (error 'setup:external-renderer "Value is not in the form '(module-path proc-id): ~v" v)]))
+                      (λ (e) (raise
+                              (exn:fail:contract (string-replace (exn-message e) "standard-module-name-resolver" "external-renderer")
+                                                 (exn-continuation-marks e))))]
+                     [exn:fail:contract? ;; raised if dynamic-require can't find render-proc-id
+                      (λ (e) (raise
+                              (exn:fail:contract (string-replace (exn-message e) "dynamic-require" "external-renderer")
+                                                 (exn-continuation-marks e))))])
+       (dynamic-require mod render-proc-id))]
+    [_ (raise-argument-error 'external-renderer "value in the form '(module-path proc-id)" v)]))
 
 ;; note that output and template order is reversed from typical
 (define (render-to-file-base caller