Define behavior for starting discussion on a deleted comment

Summary

If a user replies to an individual comment while another one deletes it, submitting the reply does not work but also shows no error (will be fixed in https://gitlab.com/gitlab-org/gitlab-ce/issues/58787). After page reload the reply is lost.

Steps to reproduce

• before https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25575 is merged: Feature.enable(:reply_to_individual_notes) in bin/rails console
• open same issue in two browser tabs
• leave a comment (not a discussion)
• start replying to comment (https://gitlab.com/gitlab-org/gitlab-ce/issues/30299)
• delete the comment in other browser tab
• submit reply

Example Project

not deployed on gitlab.com yet

What is the current bug behavior?

submitting does not work, no error shown, reply lost on page reload

What is the expected correct behavior?

an error is shown but I'm not sure what to do about the unsaved draft