MPDS-AiiDA workflow has internal check inconsistency

See https://github.com/mpds-io/mpds-aiida/blob/7d0886701cac97c03b495c059fec8ea0a963af47/mpds_aiida/workflows/crystal.py#L70

as I understand, this check is dirty and should not be there, but:

```
self.ctx.structure, error_code = self.get_structure()
if error_code is not None:
```

