s3cmd get: delete a files if object_get had failed

's3cmd get' command opens destination files as 'ab' (append+write) before even trying to download. In effect, if the
file doesn't exist, s3cmd creates it.

This patch resolves an unwanted side-effect empty files being left by s3cmd after an error.
(e.g. file not found, no permission.)

Should only delete files that s3cmd created.

Signed-off-by: Oren Held <oren@held.org.il>
Merge pull request #98 from orenhe/master

Prevent 's3cmd get' from leaving dangling empty files on failure