trace: exit silently when stdout pipe is broken

When output is not fully consumed because of piping, eg to `head`,
we get an ugly exception:

 Traceback (most recent call last):
   File "scripts/trace.py", line 274, in <module>
     args.func(args)
   File "scripts/trace.py", line 227, in list_timed
     bt_formatter(t.backtrace),
 IOError: [Errno 32] Broken pipe

Let's ignore it, it's normal condition.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>