[BE][GraphQL] Ensure mutation code is consistent with API styleguide
Any error conventions we follow should be reflected in our code.
Specifically, our standard is that we should encourage errors-as-data and enforce the following invariants:
- all mutation responses have a field named
errors
, which returns a list of errors (actual type of error is mutation dependent) - we may return errors alongside payloads. This should be indicated.
Edited by Alex Kalderimis