Changing issues confidentiality errors are not taken into account on frontend

Summary

When a user wants to change the confidentiality of an issue and the request returns an error, the user doesn't see there was a problem.

Steps to reproduce

It is possible to reproduce with confidential epic.

1. Create a confidential epic
2. Assign the confidential epic to an issue (it has to be confidential, too)
3. Go to the issue
4. Turn off the confidentiality in the side bar

What is the current bug behavior?

The user does not see any error, the confidentiality icon in the issue header is removed. After the reload, the issue is still confidential.

What is the expected correct behavior?

The user sees an error message.

Possible fixes

The response from the backend is ok. The problem needs to be fixed on frontend.

The backend response example:

[
  {
    "data":{
      "issueSetConfidential":{
        "issue":{
          "confidential":false,
          "__typename":"Issue"
        },
        "errors":[
          "Issue Cannot set confidential epic for a non-confidential issue"
        ],
        "__typename":"IssueSetConfidentialPayload"
      }
    }
  }
]