Handle broken pipes in arc_summary

Using a command similar to 'arc_summary.py | head' causes
a broken pipe exception. Gracefully exit in the case of a
broken pipe in arc_summary.py.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #6965
Closes #6969