What does this MR do and why?

Update error handling of subscription

• previously the error part of graphql never was triggered and was instead handled by errorMessages
• now error is triggered for some issues
• errorMessages has been deprecated for a more standard errors
• update tests

Changelog: changed

EE: true

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Error type	Screenshot
Error as data
Top level error

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Upload a GitLab Ultimate license
2. Navigate to a new project/group => Secure => Policies => New policy => Pipeline execution project => Create a policy
3. Verify the user is navigated to the newly created security policy project with an MR
4. Navigate to the new project/group => Secure => Policies => New policy => Pipeline execution project => Create a policy
5. Verify the user is navigated to the security policy project with an MR
6. Navigate to the new project/group => Secure => Policies => Edit policy project => Remove the linked project
7. Navigate to the new project/group => Secure => Policies => New policy => Pipeline execution project => Create a policy
8. Verify the error appears and a user can still take actions on the page
9. Repeat the steps for every policy type
10. Verify the steps above work for every policy type

Related to #495518 (closed)