Error 'failed to create fsnotify watcher: too many open files' when downloading a lot of files

During startup, an error can if a lot of files have to be downloaded:

```
time="2023-06-03T08:28:04Z" level=info msg="Retrieved 35 items from disk `aaa`"
time="2023-06-03T08:28:04Z" level=info msg="Backup definitions in 'bbb' changed, parsing new definitions."
time="2023-06-03T08:28:04Z" level=info msg="Retrieved 10 items from disk `bbb`"
2023/06/03 08:28:31 http: superfluous response.WriteHeader call from github.com/dreitier/cloudmon/web.Download (api.go:75)
2023-06-03T08:28:35.150703388Z 2023/06/03 08:28:35 http: superfluous response.WriteHeader call from github.com/dreitier/cloudmon/web.Download (api.go:75)
failed to create fsnotify watcher: too many open files
```

Assumption: s3 go client library creates temporary files when `.GetObject` is called.