Report a full_message on a worker crash

A worker crash happens very often, but we're not sure why.
I'd like to know a backtrace if it's available.

tool/lib/test/unit.rb

@@ -324,7 +324,7 @@ module Test
         return unless @options[:parallel]
         return if @interrupt
         flush_job_tokens
-        warn e if e
+        warn e.full_message if e
         real_file = worker.real_file and warn "running file: #{real_file}"
         @need_quit = true
         warn ""