Don't show failing jobs on the index page

I'm not sure quite how to treat these yet, but for now, just stick with the
previous behaviour of showing revisions which haven't been processed yet,
along with the ones that have been successfully processed.

guix-data-service/model/git-repository.scm

@@ -46,7 +46,10 @@
     "
 SELECT NULL AS id, load_new_guix_revision_jobs.id AS job_id, commit, source
 FROM load_new_guix_revision_jobs
-WHERE git_repository_id = $1 AND succeeded_at IS NULL
+WHERE git_repository_id = $1 AND succeeded_at IS NULL AND NOT EXISTS (
+  SELECT 1 FROM load_new_guix_revision_job_events
+  WHERE event = 'failure' AND job_id = load_new_guix_revision_jobs.id
+)
 UNION
 SELECT id, NULL, commit, NULL
 FROM guix_revisions