need better exception names for fgl

[Frederik]

Every once in a while, due to bugs in my code, when using fgl I see exceptions such as "Edge Exception, Node: 30". I think it would be better if messages like this in the library could be more informative, such as "Invalid node 439 in edge from node 30" or "Duplicate node 30" etc. Given that there is a problem with the graph, and given that graphs are made of nodes and edges, it is a good bet that the problem has something to do with a node or an edge, so saying "Node Exception" or "Edge Exception" doesn't provide users with very much information.

[igloo]

fgl is, according to its cabal file, maintained by Martin Erwig; contact details are on its homepage, here:  http://web.engr.oregonstate.edu/~erwig/fgl/haskell

Bug reports and feature requests should be sent to Martin.