When handling errors according to errno, catch both IOError and OSError

Different Python versions are not completely consistent about the
error that is raised and its class hierarchy:

Python 3.5.3rc1 (default, Jan  3 2017, 04:40:57)
>>> try: open('/foo')
... except Exception as e: print(e.__class__.__mro__)
(<class 'FileNotFoundError'>, <class 'OSError'>, <class 'Exception'>, <class 'BaseException'>, <class 'object'>)

Python 2.7.13 (default, Dec 18 2016, 20:19:42)
>>> try: open('/foo')
... except Exception as e: print e.__class__.__mro
(<type 'exceptions.IOError'>, <type 'exceptions.EnvironmentError'>, <type 'exceptions.StandardError'>, <type 'exceptions.Exception'>, <type 'exceptions.BaseException'>, <type 'object'>)

This can lead to a race condition during cache cleaning, where two
processes both try to delete the same file, and the one that loses
the race fails.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Iain Lane <laney@ubuntu.com>
Reviewed-by: Colin Walters <walters@verbum.org>
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=772173