Show specific error messages when adding an issue to an epic
As part of #4000, we'll show a single catch-all message for any validation error when adding an issue to an epic.
Let's provide specific messages for each kind of validation, which help the user understand the error and what they can do to solve it.
- not a properly formatted URL (e.g.
- URL is from another domain (e.g.
- URL is not of an issue (e.g.
- issue doesn't exist
- current user doesn't have editing permissions for the issue
- issue belongs to a project outside of the group or its subgroups