s3cmd get to stdout: set file_exists to avoid traceback, bug #175

Fixes https://github.com/s3tools/s3cmd/issues/175
Reported by Seth Noble.

Set file_exists so we don't try to delete sys.stdout if getting the
file happens to fail.
Merge pull request #176 from mdomsch/bug/175

s3cmd get to stdout: set file_exists to avoid traceback, bug #175