Better error messages

[PythonFZ]

train_selection = val_selection.excluded_atoms

val_data += val_selection.atoms
train_data += train_selection #.atoms raises error

have something like: znflow.ConnectionError: connected attribute "list" has no attribute "atoms"

instead of:

TypeError: Can not add <class 'ipsuite.configuration_selection.random.RandomSelection'> to <class 'znflow.base.Connection'>.

and

AttributeError: 'Connection' object has no attribute 'atoms'