Unwind the stack when handling transaction errors

As this avoids trying to suspend while the stack is in a unpredictable state.

guix-data-service/database.scm

@@ -216,13 +216,15 @@
           (lambda ()
             (exec-query conn "ROLLBACK;"))
           #:unwind? #t)
+        ;; TODO Include the stack in the exception via knots
         (raise-exception exn))
     (lambda ()
       (let ((result (f conn)))
         (exec-query conn (if always-rollback?
                              "ROLLBACK;"
                              "COMMIT;"))
-        result))))
+        result))
+    #:unwind? #t))
 
 (define (check-test-database! conn)
   (match (exec-query conn "SELECT current_database()")