Fix bug in explore-attributes

if given some background knowledge, starting with the empty
set as first premise is not correct.  Instead, we should
start with the closure of the empty set, which is what we
do now.

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>
Make exploration abortable

During exploration, if the expert throws an exception,
explore-attributes returns the results obtained so far

Signed-off-by: Daniel Borchmann <daniel.borchmann@mailbox.tu-dresden.de>