Then, client apps can show the error messages to end users, so they might correct the right fields in a form, for example.

In general, top-level errors should only be used for exceptional circumstances when a developer should be made aware that the system had some kind of problem. For example, the GraphQL specification says that when a non-null field returns nil, an error should be added to the "errors" key. This kind of error is not recoverable by the client. Instead, something on the server should be fixed to handle this case. When you want to notify a client some kind of recoverable issue, consider making error messages part of the schema, for example, as in mutation errors.