raise an error when no energy found

if no energy is found (not in the .nx or in the .info file) we end up with the following error:

------------- stderr -------------
/nobackup/lid16nagpu11/tomotools/envs/2022_12_12/lib/python3.8/site-packages/skcuda/cublas.py:284: UserWarning: creating CUBLAS context to get version number
  warnings.warn('creating CUBLAS context to get version number')
Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/nobackup/lid16nagpu11/tomotools/envs/2022_12_12/lib/python3.8/site-packages/nabu/app/reconstruct.py", line 156, in <module>
    main()
  File "/nobackup/lid16nagpu11/tomotools/envs/2022_12_12/lib/python3.8/site-packages/nabu/app/reconstruct.py", line 102, in main
    if proc.dataset_info.energy < 1e-3 and proc.nabu_config["phase"]["method"] is not None:
TypeError: '<' not supported between instances of 'NoneType' and 'float'

------------- stdout -------------

instead it would be good to have something like a raise ValueError("energy missing")