IMPALA-2912: failure in BBMT::RuntimeStateTeardownWriteError
IMPALA-2912: failure in BBMT::RuntimeStateTeardownWriteError

The test would intermittently fail because Block::Unpin() returned
cancelled. This is expected behaviour, since a previous Unpin() call may
cause a write error that causes the block manager to be cancelled.

This patch updates the test to allow Block::Unpin() to return cancelled
in this situation.

Change-Id: I9ebcfbd7ba64260f9e6652ec0eee371155d99b1f
Reviewed-on: http://gerrit.cloudera.org:8080/1977
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Internal Jenkins

kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala,kapilrastogi/Impala