Better report exceptions in query-build-server-substitutes

guix-data-service/substitutes.scm

@@ -53,7 +53,7 @@
                     (member id build-server-ids))
             (when lookup-all-derivations?
               (simple-format #t "\nQuerying ~A\n" url)
-              (catch #t
+              (with-throw-handler #t
                 (lambda ()
                   (fetch-narinfo-files conn id url revision-commits
                                        #:specific-outputs
@@ -62,7 +62,8 @@
                   (simple-format
                    (current-error-port)
                    "exception in query-build-server: ~A ~A\n"
-                   key args)))))))
+                   key args)
+                  (backtrace)))))))
        build-servers))))
 
 (define %narinfo-max-size