Fallback on checksum jobs

Since `calculate_checksum` depends on `Uploader` classes which are not defined in this background migration and may change at any time.

lib/gitlab/background_migration/populate_untracked_uploads.rb

@@ -166,6 +166,8 @@ module Gitlab
           return unless exist?
           self.checksum = Digest::SHA256.file(absolute_path).hexdigest
+        rescue StandardError
+          schedule_checksum
         end
         def exist?