Remove graph.Err helper function

Since Err() is now a member of the Iterator interface, we don't need
this helper function anymore.

graph/iterator.go

@@ -177,17 +177,6 @@ func Next(it Iterator) bool {
 	return false
 }
 
-// Err is a convenience function that conditionally calls the Err method
-// of an Iterator if it is a Nexter.  If the Iterator is not a Nexter, Err
-// returns nil.
-func Err(it Iterator) error {
-	if n, ok := it.(Nexter); ok {
-		return n.Err()
-	}
-	glog.Errorln("Calling Err on an un-nextable iterator")
-	return nil
-}
-
 // Height is a convienence function to measure the height of an iterator tree.
 func Height(it Iterator, until Type) int {
 	if it.Type() == until {