SPARK-2738. Remove redundant imports in BlockManagerSuite

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1642 from sryza/sandy-spark-2738 and squashes the following commits:

a923e4e [Sandy Ryza] SPARK-2738. Remove redundant imports in BlockManagerSuite
[SPARK-2670] FetchFailedException should be thrown when local fetch has failed

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #1578 from sarutak/SPARK-2670 and squashes the following commits:

85c8938 [Kousuke Saruta] Removed useless results.put for fail fast
e8713cc [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-2670
d353984 [Kousuke Saruta] Refined assertion messages in BlockFetcherIteratorSuite.scala
03bcb02 [Kousuke Saruta] Merge branch 'SPARK-2670' of github.com:sarutak/spark into SPARK-2670
5d05855 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-2670
4fca130 [Kousuke Saruta] Added test cases for BasicBlockFetcherIterator
b7b8250 [Kousuke Saruta] Modified BasicBlockFetchIterator to fail fast when local fetch error has been occurred
a3a9be1 [Kousuke Saruta] Modified BlockFetcherIterator for SPARK-2670
460dc01 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-2670
e310c0b [Kousuke Saruta] Modified BlockFetcherIterator to handle local fetch failure as fatch fail