BUG-865: do not use InstanceIdentifier.getPath()

The method was only used to speed up an ArrayList, which in turn was only
ever used for debugging. Instead of using a precodintion which forces us
to do this, just track the offset of the offending entry. If we find
ourselves in the error territory, we can then take getPathArguments(),
limit it to proper size and print it out.

Change-Id: I3c1786fe77cef005a6463c22b5be8bb8672c26bb
Signed-off-by: Robert Varga <rovarga@cisco.com>