[SPARK-9262][build] Treat Scala compiler warnings as errors

I've seen a few cases in the past few weeks that the compiler is throwing warnings that are caused by legitimate bugs. This patch upgrades warnings to errors, except deprecation warnings.

Note that ideally we should be able to mark deprecation warnings as errors as well. However, due to the lack of ability to suppress individual warning messages in the Scala compiler, we cannot do that (since we do need to access deprecated APIs in Hadoop).

Most of the work are done by ericl.

Author: Reynold Xin <rxin@databricks.com>
Author: Eric Liang <ekl@databricks.com>

Closes #7598 from rxin/warnings and squashes the following commits:

beb311b [Reynold Xin] Fixed tests.
542c031 [Reynold Xin] Fixed one more warning.
87c354a [Reynold Xin] Fixed all non-deprecation warnings.
78660ac [Eric Liang] first effort to fix warnings