Use GraphQL errors_on_object for collecting errors

What does this MR do?

Around 50% of our mutations were calling object.errors.full_messages within their #resolve method, while others were using the #errors_on_object helper in BaseMutation.

This MR changes all mutations to use #errors_on_object, and updates the GraphQL API developer docs to mention the helper so it has some visibility.

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.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done