Bug 1363: Introduced ModifiedNodeDoesNotExistsException.

Introduced new checked exception ModifiedNodeDoesNotExists.
This exception is thrown when node which has subtree
modification did not exist at time of transaction allocation
and also do not exist when allocation was processed
to be commited.

Change-Id: I54f040034b43bfaf4bdda089ad9d2378ddd6ab09
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>