graph.py: add try/catch for computing synapse risk
graph.py: add try/catch for computing synapse risk

For an unknown reason, rarely an edge fails.

fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID