Nonbonded potentials silently ignored

If the (unnecessary) parametrisation of the hardsphere potential is omitted, i.e., ``- hardsphere``, the potential is silently ignored, cf. ``- hardsphere: {}``. An error  shall be raised and a user help shall be provided if no parametrisation is provided.
```
energy:
- nonbonded_exact:
    default:
    - hardsphere
```