if there are no edges, then raise an error for edge type in multigraph

[RazinShaikh]

I think this should raise an error but for the simple graph, the current code returns 0.

pyzx/pyzx/graph/graph_s.py

Lines 272 to 277 in e8a0de5

	def edge_type(self, e):
	v1,v2 = e
	try:
	return self.graph[v1][v2]
	except KeyError:
	return 0

If you want, I can change it to returning 0 for multigraph as well but that seems odd to me because 0 is the same as simple type.