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 (closed).

Screenshots (if relevant)

UPDATED:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together