Return meaningful error when adding one child epic at time and it fails

What does this MR do?

When adding a child epic to a parent fails, a general message is displayed in most cases. (Exceptions are when the parent is already too deep and when an epic already exists among the parent ancestors).

One of the reasons why it is like that is that we support adding multiple child epics at once.

This is the first iteration towards improving user experience.

If a user adds a new epic as a child and it fails a meaningful, case-specific, message will be displayed. The behavior for adding multiple messages does not change. The behavior of other related classes does not change either, although we could consider improving the user experience there as well.

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.

Issue: #35546 (closed)