Unhelpful error in `depthNumbering`

[AndreasVoellmy]

If you apply depthNumbering starting from a root vertex that is not in the graph, you get the error:

Data.Graph.Wrapper.fromList: one of the edges of a vertex pointed to a vertex that was not supplied in the input

This suggests that the problem is fromList. However, the problem is really caused by a call to indexGVertex in depthNumbering. fromList has nothing to do with the problem.

I'm not sure what the ideal resolution is... at the least, it might be good to not mention fromList in the error message.

[soenkehahn]

I'm maintaining graph-wrapper now: https://github.com/soenkehahn/graph-wrapper
But I guess we can keep this issue here.

I think depthNumbering should just ignore non-existing roots. @AndreasVoellmy: Would you agree?

(cc: @batterseapower)

[soenkehahn]

I've created a PR: soenkehahn#2