Ensure sys.exit called in fork_child after exception

Currently, the fork_child() function in auditor.py does not
handle the case where run_audit() encounters an exception
properly.

A simple case is where the /srv directory is set
with permissions such that the 'swift' user cannot access it.
Such a situation causes a os.listdir() to return an OSError
exception.  When this happens the fork_child() process does not
run to completion and sys.exit() is not executed.  The process
that was forked off continues to run as a result.  Execution goes
back up to the audit_loop function which restarts the whole process.  The
end result is an increasing number of processes on the system
until the parent is terminated.  This can quickly exhaust the
process descriptors on a system.

This change wraps run_audit() in a try block and adds an
exception handler that prints what exception was encountered.
The sys.exit() was moved to a finally: block so that it will
always be run, avoiding the creation of zombies.

Change-Id: I89d7cd27112445893852e62df857c3d5262c27b3
Closes-bug: 1375348
(cherry picked from commit 301a96f664d58b4ccad8e3cbf5d5a889cc76790f)