Fix BackupService crashing when an attachment is missing (#11241)

* Fix BackupService crashing when an attachment is missing

For various reasons such as admin error or out-of-sync media and
database backups, it might be possible for local attachments to be lost.

This commit allows the BackupService to continue its work even if some media
file is missing.

* Change error message

app/services/backup_service.rb

@@ -142,5 +142,7 @@ class BackupService < BaseService
         io.write(buffer)
       end
     end
+  rescue Errno::ENOENT
+    Rails.logger.warn "Could not backup file #{filename}: file not found"
   end
 end