Protect against hash cleanup errors on PUTs

In http://launchpad.net/bugs/1254405 an exception occurs when
finalizing the PUT of an object, but it is obscured by the thread pool
code, so we don't see the actual line where it originated. However,
there are two possible functions where this exception could originate:

    1. renamer()
    2. hash_cleanup_listdir()

If this is an error in renamer(), there is some other waky problem
where the temporary file has been removed. It is likely that this is a
problem where a file name from os.listdir ends up disappearing (but
even this is a rare occurence). Regardless, it is not clear that we
really want an error from hash_cleanup_listdir() from affecting the
return result of the PUT.

To that end, we squelsh OSErrors from hash_cleanup_listdir() for
now. One might argue for all errors, but since os.unlink() and
os.listdir() raise OSError today, that is probably sufficient.

Even that we use "Closes-Bug" below, it is not clear it can even be
determined that this closes that bug report.

Change-Id: I2f55df835c387e4d17cffda74c04c9994aebbe1f
Closes-Bug: 1254405
Signed-off-by: Peter Portante <peter.portante@redhat.com>