Prepare GraphQL resolvers for authorization framework changes

What does this MR do?

This MR includes changes necessary to prepare our tests for the new authorization framework that will have externally observable effects on behaviour.

Specifically two changes are included:

• some resolvers that would previously raise errors for unauthorized resources will now return nil
• some resolvers that would previously raise internal server errors, will now raise specific errors

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team