Backward compatibility of the `clean_button` and exception catching

[mikibonacci]

With PR , I introduced the clean_button for remote data. However, I added some backward incompatible thing, wrt to AiiDA core < 2.6, i.e. the usage of is_cleaned to check if the remote is empty. I need to put back the is_empty attribute.

Also, if I except with (OSError, KeyError) as exceptions, the app will except too. This is not really clear to me, but seems to be fixed just avoiding to put OSError.