Stop using exception handling when inserting blocked_builds entries

As it doesn't work in a transaction.

guix-data-service/model/blocked-builds.scm

@@ -174,13 +174,8 @@ ON CONFLICT DO NOTHING")
      '()))
 
   (unless (null? data)
-    (with-exception-handler
+    (create-partitions)
-        (lambda (exn)
+    (try-insert)))
-          (create-partitions)
-
-          (try-insert))
-      try-insert
-      #:unwind? #t)))
 
 (define (handle-populating-blocked-builds-for-scheduled-builds conn build-ids)
   (define (get-build-details build-id)