Missing error handling for /epic quick action

Summary

/epic always displays Added an issue to an epic. even if the corresponding epic does not exist or the user does not have sufficient permission.

Steps to reproduce

• use /epic on issue with a non-existing epic

What is the current bug behavior?

Added an issue to an epic. is shown.

What is the expected correct behavior?

• This epic does not exist or you don't have sufficient permission. is shown.

Possible fixes

Check result in ee/lib/ee/gitlab/quick_actions/issue_actions.rb and adjust @execution_message.