Unavailable quick actions fail silently

Summary

Running slash commands that user isn't authorized to run gives no visual feedback.

Steps to reproduce

1. Visit nick.volynkin/gitlab-ce#1
2. Run /assign @nick.volynkin in a comment.
3. (If you aren't an admin) the command will fail but give no visual feedback.

Expected behavior

• Successfull command shows a message with green background. Failed one might show an error message with red background
• If submitted by email, should generate a reply with error message
• If submitted by API, should return an error message (haven't checked this).

Relevant logs and/or screenshots

Can't screenshot a missing message. :)

Output of checks

(Looks like this doesn't apply)

Results of GitLab Application Check

Results of GitLab Environment Info

Possible fixes