Make BackupService resilient to read timeouts

If an attachment read times out, assume that the resources is
inaccessible and continue the backup without it. This fixes #12280.

app/services/backup_service.rb

@@ -143,6 +143,7 @@ class BackupService < BaseService
       end
     end
   rescue Errno::ENOENT
+  rescue Seahorse::Client::NetworkingError
     Rails.logger.warn "Could not backup file #{filename}: file not found"
   end
 end