[SPARK-9710] [TEST] Fix RPackageUtilsSuite when R is not available.

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.

core/src/main/scala/org/apache/spark/api/r/RUtils.scala

@@ -67,7 +67,11 @@ private[spark] object RUtils {
 
   /** Check if R is installed before running tests that use R commands. */
   def isRInstalled: Boolean = {
-    val builder = new ProcessBuilder(Seq("R", "--version"))
-    builder.start().waitFor() == 0
+    try {
+      val builder = new ProcessBuilder(Seq("R", "--version"))
+      builder.start().waitFor() == 0
+    } catch {
+      case e: Exception => false
+    }
   }
 }