Hacked in a better message for attachment network error
## What does this MR do?
Adds a better error message for when a user is uploading an attachment and they cannot reach the GitLab instance.
Now displays `Network error. Please check your connection.`.
## Are there points in the code the reviewer needs to double check?
As noted above, I think this is hacky, ideally this would be sent straight from the backend but if not then this should be handled globally?
/cc @fatihacet
## Why was this MR needed?
Attachment upload UX
## What are the relevant issue numbers?
Closes#19161.
## Screenshots (if relevant)
*UPDATED:*
## Does this MR meet the acceptance criteria?
- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
- [ ] Added for this feature/bug
- [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
See merge request !4977
6d12878d5a