Fix issue with handling failure to build system channel instance

That resulted in empty revisions. Raise an exception rather than returning #f.

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

@@ -2239,7 +2239,13 @@ WHERE builder != 'builtin:download'
           (assoc-ref channel-derivations-by-system
                      (%current-system))
           'manifest-entry-item)))
-    (if derivation-file-name-for-current-system
+
+    (unless derivation-file-name-for-current-system
+      (raise-exception
+       (make-exception-with-message
+        (simple-format #f "Failed to generate store item for ~A\n"
+                       commit))))
+
     (let ((derivation-for-current-system
            (read-derivation-from-file derivation-file-name-for-current-system)))
       (with-time-logging "building the channel derivation"
@@ -2249,8 +2255,7 @@ WHERE builder != 'builtin:download'
 
       (values
        (derivation->output-path derivation-for-current-system)
-           derivation-file-name-for-current-system))
-        (values #f #f))))
+       derivation-file-name-for-current-system))))
 
 (prevent-inlining-for-tests channel-derivations-by-system->guix-store-item)
 
@@ -2859,7 +2864,7 @@ WHERE builder != 'builtin:download'
   (destroy-resource-pool inf-and-store-pool)
   (destroy-resource-pool postgresql-connection-pool)
 
-  #t)
+  *unspecified*)
 
 (prevent-inlining-for-tests extract-information-from)
 
@@ -3027,8 +3032,7 @@ WHERE builder != 'builtin:download'
             guix-derivation
             (channel-derivations-by-system->guix-store-item
              channel-derivations-by-system)))
-      (if store-item
-          (and
+
       (extract-information-from conn
                                 guix-revision-id-promise
                                 commit guix-source store-item
@@ -3044,6 +3048,7 @@ WHERE builder != 'builtin:download'
                                 #:parallelism parallelism
                                 #:inferior-memory-limit
                                 inferior-memory-limit)
+
       (let ((guix-revision-id
              (fibers-force guix-revision-id-promise)))
         (destroy-parallelism-limiter
@@ -3051,7 +3056,6 @@ WHERE builder != 'builtin:download'
         (destroy-thread-pool
          utility-thread-pool)
 
-             (and
         (if (defined? 'channel-news-for-commit
               (resolve-module '(guix channels)))
             (with-time-logging "inserting channel news entries"
@@ -3059,31 +3063,28 @@ WHERE builder != 'builtin:download'
                conn
                guix-revision-id
                (channel-news-for-commit channel-for-commit commit)))
-                  (begin
             (simple-format
-                     #t "debug: importing channel news not supported\n")
-                    #t))
+             #t "debug: importing channel news not supported\n"))
 
         (with-time-logging "updating builds.derivation_output_details_set_id"
           (update-builds-derivation-output-details-set-id
            conn
            guix-revision-id))
+
         (update-package-derivations-table
          conn
          git-repository-id
          guix-revision-id
          commit)
+
         (let ((stats (gc-stats)))
           (format (current-error-port)
                   "gc-stats: time taken: ~3fs, times: ~d~%"
                   (/ (assq-ref stats 'gc-time-taken)
                      internal-time-units-per-second)
-                        (assq-ref stats 'gc-times))
-                #t))))
-          (begin
-            (simple-format #t "Failed to generate store item for ~A\n"
-                           commit)
-            #f)))))
+                  (assq-ref stats 'gc-times))))))
+
+  *unspecified*)
 
 (define (enqueue-load-new-guix-revision-job conn git-repository-id commit source)
   (define query