MM-38702: Do not fail during importing without attachment (#20800)

We just log a warning instead of failing completely.

https://mattermost.atlassian.net/browse/MM-38702

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

i18n/en.json

@@ -4875,10 +4875,6 @@
     "id": "app.import.bulk_import.json_decode.error",
     "translation": "JSON decode of line failed."
   },
-  {
-    "id": "app.import.bulk_import.process_attachments.error",
-    "translation": "Error while processing bulk import attachments."
-  },
   {
     "id": "app.import.bulk_import.unsupported_version.error",
     "translation": "Incorrect or missing version in the data import file. Make sure version is the first object in your import file and try again."