Ensure thread_pool.join runs even when an exception is raised

Move thread_pool.join and job stats reporting into an ensure block
so that all threads are properly joined before rake exits, even when
a task raises an exception. Previously, an exception in a multitask
prerequisite could cause rake to exit while subprocesses were still
running.

Fixes #414

Author: hsbt

lib/rake/application.rb

@@ -146,7 +146,7 @@ def run_with_threads
       thread_pool.gather_history if options.job_stats == :history
 
       yield
-
+    ensure
       thread_pool.join if defined?(@thread_pool)
       if options.job_stats
         stats = thread_pool.statistics