MB-11349 report compaction error to memcached

return code from notifyCompaction was masking error in
actual compaction. Fix to return FAILED message to memcached

Change-Id: Iae5b88daec56c2adbf86709557d279a029599535
Reviewed-on: http://review.couchbase.org/38032
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>