Stop logging complete data tree on prepare/commit failure

Sometimes the data tree modification is so large that just trying to create
the buffer to hold the message can make the controller run out of memory. Plus
it's rarely useful to have a log filled with data which obfuscates other
important log messages. This patch still logs the data tree modification at
trace level.

Change-Id: I76bff9f7e836ee5eff347b0b77e2817f441ab953
Signed-off-by: Moiz Raja <moraja@cisco.com>