bug#70613: Report GraphQL error when queried for non-existent bug

[Arun Isaac]

When the mumi GraphQL endpoint is requested for a non-existent bug, it
should provide a clear error message in the response. At the moment, it
merely fails with an "Internal Server Error", and the user is left
groping in the darkness.

Here's a minimal failing example:

--8<---------------cut here---------------start------------->8---
(use-modules (kolam http)
             (ice-9 pretty-print))

(define %graphql-endpoint
  "https://issues.guix.gnu.org/graphql";)

(pretty-print
 (graphql-http-get %graphql-endpoint
                   '(document
                     (query (#(issue #:number 2)
                             number
                             title
                             date
                             open)))))
--8<---------------cut here---------------end--------------->8---