Fix the output of cta-prod-show-dataset

Fix the cta-prod-show-dataset output for the case where the dataset does not exist.

Currently we get:

$ cta-prod-show-dataset Prod6_Paranal_Prod6-Hyperarray_NSB1x_gamma-diffuse_North_20deg_0.008div_R1_test

Available datasets: 1

Prod6_Paranal_Prod6-Hyperarray_NSB1x_gamma-diffuse_North_20deg_0.008div_R1_test


Storage usage for all datasets

|_. Name |_. N files |_. Size(TB) |
Traceback (most recent call last):
  File "/Users/arrabito/ctadirac_client/diracos/bin/cta-prod-show-dataset", line 8, in <module>
    sys.exit(main())
  File "/Users/arrabito/ctadirac_client/diracos/lib/python3.9/site-packages/DIRAC/Core/Base/Script.py", line 74, in __call__
    return entrypointFunc._func()
  File "/Users/arrabito/PycharmProjects/CTADIRAC/src/CTADIRAC/Core/scripts/cta_prod_show_dataset.py", line 226, in main
    name, n_files, size, mq = get_dataset_info(dataset_name)
  File "/Users/arrabito/PycharmProjects/CTADIRAC/src/CTADIRAC/Core/scripts/cta_prod_show_dataset.py", line 49, in get_dataset_info
    res = dataset_dict['Successful'][dataset_name][dataset_name]
KeyError: 'Prod6_Paranal_Prod6-Hyperarray_NSB1x_gamma-diffuse_North_20deg_0.008div_R1_test'