RUtils.isRInstalled throws an exception if R is not installed,
instead of returning false. Fix that.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #8008 from vanzin/SPARK-9710 and squashes the following commits:

df72d8c [Marcelo Vanzin] [SPARK-9710] [test] Fix RPackageUtilsSuite when R is not available.