Less noise on a partest failure.

Throwing a BuildException is the polite way to fail the Ant build.

Before:

	BUILD FAILED
	/Users/jason/code/scala2/build.xml:1522: java.lang.RuntimeException: Test suite finished with 1 case failing:
	/Users/jason/code/scala2/test/files/pos/lub-dealias-widen.scala [FAILED]
	        at scala.sys.package$.error(package.scala:27)
	        [20 lines elided]
	        at org.apache.tools.ant.launch.Launcher.main(Launcher.java:109)

	Total time: 2 minutes 35 seconds

After:

	BUILD FAILED
	/Users/jason/code/scala2/build.xml:1522: Test suite finished with 1 case failing:
	/Users/jason/code/scala2/test/files/pos/lub-dealias-widen.scala [FAILED]

	Total time: 2 minutes 34 seconds
Merge pull request #2686 from retronym/topic/partest-task-no-stack-trace

Less noise on a partest failure.