Resolve "Display a better message when starting a discussion on a deleted comment"

What does this MR do?

When starting a discussion on a deleted comment fails shows a meaningful message: Your comment could not be submitted because discussion to reply cannot be found.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Related #27239 (closed)