zkPython's NodeExistsException should include information about the node that exists

Currently the code creates a zookeeper.NodeExistsException object with a string argument "node exists".

Including the name of the node that caused the exception would be useful, in that it allows user code like the following:

try:
  zookeeper.create(zh, n1, ...)
  zookeeper.create(zh, n2, ...)
except zookeeper.NodeExistsException, n:
  print "Node \"%s\" exists." % n