Fix unwanted silencing of S3Error when file_exists

S3Error should always be raised, regardless of the value of file_exists.

Bug reproducible when using get --continue while the target file exists.
The issue was recently introduced by commit 1a051563e.
Thanks @jamiew for noticing the issue.
Merge pull request #110 from orenhe/master

s3cmd get --continue: fix unwanted silencing of S3Error exception when file exists.