Complete the os.path.exists() cleanups from 1.0.3 to all os.path.exists() calls.

Test Plan:
Fingers crossed.

# --
# For 'Test Plan:', list the commands you ran or process you
#   followed to test this change.
# If this commit is adding to an existing review, delete the
# 'Test Plan:' lines and replace them with 'Review: Kxxx'.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch exists
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#	modified:   polib.py
#

Reviewers: john, chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D2266