Update error handling of subscription

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

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MR acceptance checklist

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Screenshots or screen recordings

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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)