Fix marking jobs as failed when exceptions are raised

The switch away from catch broke this, I obviously still don't quite get how
with-exception-handler works. Therefore, use it twice as that seems to help.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -1592,6 +1592,9 @@ SKIP LOCKED")
               (with-time-logging (string-append "loading revision " commit)
                 (setup-logging
                  id
+                 (lambda ()
+                   (with-exception-handler
+                       (const #f)
                      (lambda ()
                        (with-exception-handler
                            (lambda (exn)
@@ -1606,7 +1609,8 @@ SKIP LOCKED")
                               (load-new-guix-revision conn
                                                       store
                                                       git-repository-id
-                                                  commit))))))))
+                                                      commit))))))
+                     #:unwind? #t))))
               #t))
             (begin
               (record-job-succeeded conn id)