[BUG] Fix handling of fromstring() for invalid enums

[parthosa]

Currently we depend on EnumeratedType.fromstring() to manage invalid enum arguments by raising an exception. However, it returns None for invalid values, instead of throwing an exception. Thus the intended default value, which is set in the exception handling branch, is never applied.

For example,

try:
  selected_recommendation = QualGpuClusterReshapeType.fromstring(arg_val)
except Exception:  # pylint: disable=broad-except
  => execution never reaches here in case of invalid enum
  selected_recommendation = QualGpuClusterReshapeType.fromstring(default_recommendation_txt)