explicit return if motif cannot be found

[marioangst]

We should have a return (it would make sense if this was returned from sma) if a motif cannot be found in a given network.

This would make for a much better UI in show_motif and explore_motifs, because we could then throw an explicit error message (eg. "the motif(s) xxx do not exist in the network supplied")

[tseppelt]

With the next version of sma (2.1.2, currently being uploaded to PyPI), exemplify_motif will return NULL instead of raising an error:

 > exemplify_motif(directed_dummy_net, "2,1[III.C.6]")
NULL

[marioangst]

wrapped up in 400400b0940e7b8243adf65dafd32714a2647d79