Suppress error message during failed UmfpackContext import 

This message was caused by a bug in scipy that was fixed by @tonysyu after 0.12. Since we probably don't want to bump up the scipy requirement that high, this is probably a useful thing to do for scikit-image users.

See discussion in 
https://groups.google.com/d/msg/scikit-image/FrM5IGP6wh4/1hp-FtVZmfcJ

and solution proposed in
http://stackoverflow.com/questions/13977970/ignore-exceptions-printed-to-stderr-in-del/13977992?noredirect=1#comment28386412_13977992

@JDWarner proposes that we use the priority pyamg -> umf -> brute force for random walker. I'll let others decide whether we want that in this release. But I think suppressing the error message at least is a very nice thing to include.