Monkey-patch UmfpackContext __del__

By putting the failing statement in a try: except: clause, the
exception is caught and the error message is silenced.

See
https://groups.google.com/d/msg/scikit-image/FrM5IGP6wh4/1hp-FtVZmfcJ
and
http://stackoverflow.com/questions/13977970/ignore-exceptions-printed-to-stderr-in-del/13977992?noredirect=1#comment28386412_13977992

skimage/segmentation/random_walker_segmentation.py

@@ -9,14 +9,25 @@ significantly the performance.
 """
 
 import warnings
+import sys
+import os
 
 import numpy as np
 from scipy import sparse, ndimage
+
 try:
     from scipy.sparse.linalg.dsolve import umfpack
+    old_del = umfpack.UmfpackContext.__del__
+    def new_del(self):
+        try:
+            old_del(self)
+        except AttributeError:
+            pass
+    umfpack.UmfpackContext.__del__ = new_del
     UmfpackContext = umfpack.UmfpackContext()
 except:
     UmfpackContext = None
+
 try:
     from pyamg import ruge_stuben_solver
     amg_loaded = True