Raise an exception on missing level counts

guix-data-service/model/guix-revision-package-derivation.scm

@@ -18,6 +18,7 @@
 (define-module (guix-data-service model guix-revision-package-derivation)
   #:use-module (ice-9 match)
   #:use-module (ice-9 threads)
+  #:use-module (ice-9 exceptions)
   #:use-module (squee)
   #:use-module (guix-data-service database)
   #:use-module (guix-data-service utils)
@@ -301,6 +302,11 @@ WITH l0 AS (
      "
 )"))
 
-  (if level-counts
-      (query level-counts)
-      #f))
+  (if (null? level-counts)
+      (raise-exception
+       (make-exception-with-message
+        (simple-format #f "missing level counts for ~A ~A ~A"
+                       guix-revision-id
+                       system-id
+                       target)))
+      (query level-counts)))