KUDU-111 Always set the failure Status on Transaction::HandlePrepareFailure
KUDU-111 Always set the failure Status on Transaction::HandlePrepareFailure

Instead of just checking that the error was set, HandlePrepareFailure now
actually sets the error on the TransactionCompletedCallback. The error is
set in only one place on Prepare() failure and one place on Apply() failure.
This also makes HandlePrepareFailure() answer the client if the Commit() failed.

Change-Id: I2a84e04540de42cef59ca92d6fd0d857b3e6423c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1524
Tested-by: jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

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