Fix ambiguous interface for creating perturbed ensembles

To run a perturbed ensemble in ICON, one can:

a. Give values to `pinit_seed`/`pinit_amp` when calling `run-ensemble` command (ICON builtin perturb functionality). If `pinit_seed` is not equal to 0, this will trigger the functions `add_random_noise_*` in [icon/src/drivers/mo_atmo_nonhydrostatic.f90](https://gitlab.dkrz.de/icon/icon-nwp/-/blob/master/src/drivers/mo_atmo_nonhydrostatic.f90?ref_type=heads) and makes use of `pinit_amp` to pertub the prognostic variables `w`, `vn`, `theta_v`, `exner` and `rho`. Depending on other namelist switches, additional variables may be perturbed.
b. Make several copies of a first "ref" netcdf file and perturb them via `perturb` command (non-icon runs without a builtin perturbation functionality like the granules need this option)

This interface is ambiguous, and should be clarified/fixed.

Extra 1: make fields necessary for creating the ensemble file directories "required"
Extra 2: add error checks in case files/directories are not created/copied

Start by tracing the run-ensemble and perturb command interfaces, then read the linked ICON driver file to understand the built-in perturbation path. Clarify which inputs are required for ensemble-file directories and define error checks for files or directories that are not created or copied. Done means the two perturbation workflows are unambiguous and these validation failures are reported.

Written by the indexing model from the issue text.