Don't discard data in case of an error

GraphQL allows to pass both, `data` as well as `error` entries. It would be nice it this would be posssible here. For example, for bulk operations that would help to report partial sucesses alongside with the errors in a proper manner.

No file or test is named. Start by tracing how execution results and errors are represented and serialized, then define tests covering responses that contain both data and errors, including partial success for bulk operations; done means those results are preserved and reported together.

Written by the indexing model from the issue text.