Add more debug message for ManagedBuffer

This is to help debug the error reported at http://apache-spark-user-list.1001560.n3.nabble.com/SQL-queries-fail-in-1-2-0-SNAPSHOT-td15327.html

Author: Reynold Xin <rxin@apache.org>

Closes #2580 from rxin/buffer-debug and squashes the following commits:

5814292 [Reynold Xin] Logging close() in case close() fails.
323dfec [Reynold Xin] Add more debug message.
Merge branch 'master' into netty-blockTransferService

Conflicts:
	core/src/main/scala/org/apache/spark/network/ManagedBuffer.scala