Merge pull request #98 from orenhe/master

Prevent 's3cmd get' from leaving dangling empty files on failure
Fix unwanted silencing of S3Error in case file_exists

Bug reproducible when using get --continue while the target file exists.

The issue was recently introduced by 1a051563e. S3Error should always be thrown in this case regardless of file_exists
value.

Thanks @jamiew for noticing the issue.