Fix backups stopping due to read timeouts (mastodon#12281)

* 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 mastodon#12280.

* Both errors on one line

app/services/backup_service.rb

@@ -166,7 +166,7 @@ def download_to_tar(tar, attachment, filename)
         io.write(buffer)
       end
     end
-  rescue Errno::ENOENT
+  rescue Errno::ENOENT, Seahorse::Client::NetworkingError
     Rails.logger.warn "Could not backup file #{filename}: file not found"
   end
 end