Try to address the issue of missing derivation outputs

guix-data-service/model/derivation.scm

@@ -21,6 +21,7 @@
   #:use-module (ice-9 vlist)
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
+  #:use-module (ice-9 exceptions)
   #:use-module (ice-9 binary-ports)
   #:use-module (rnrs bytevectors)
   #:use-module (squee)
@@ -73,7 +74,12 @@
             update-derivation-inputs-statistics
             vacuum-derivation-inputs-table
             update-derivation-outputs-statistics
-            vacuum-derivation-outputs-table))
+            vacuum-derivation-outputs-table
+
+            &missing-derivation-output-error
+            missing-derivation-output-error?
+            missing-derivation-output-error-name
+            missing-derivation-output-error-path))
 
 (define (valid-targets conn)
   '("arm-linux-gnueabihf"
@@ -1128,12 +1134,19 @@ ON CONFLICT DO NOTHING"
                 (vector->list (json-string->scm env_vars)))
            system))))
 
+(define-exception-type &missing-derivation-output-error &error
+  make-derivation-output-error
+  missing-derivation-output-error?
+  (name missing-derivation-output-error-name)
+  (path missing-derivation-output-error-path))
+
 (define select-derivation-output-id
   (mlambda (conn name path)
     (match (exec-query
             conn
             "
-SELECT derivation_outputs.id FROM derivation_outputs
+SELECT derivation_outputs.id
+FROM derivation_outputs
 INNER JOIN derivations
   ON derivation_outputs.derivation_id = derivations.id
 WHERE derivations.file_name = $1
@@ -1143,9 +1156,8 @@ WHERE derivations.file_name = $1
       (((id))
        id)
       (()
-       (error (simple-format
-               #f "cannot find derivation-output with name ~A and path ~A"
-               name path))))))
+       (raise-exception
+        (make-derivation-output-error name path))))))
 
 (define (select-derivation-outputs-by-derivation-id conn id)
   (define query