Backup scripts don't handle S3 Errors

[toffer]

Rare, intermittent "BadRequest" errors leads to corrupted backup on S3.

Here's an example error message from running msm-backup-working-files-to-s3.py when it calls boto_rsync:

boto.exception.S3ResponseError: S3ResponseError: 400 Bad Request
<?xml version="1.0" encoding="UTF-8"?>
<Error>
    <Code>BadDigest</Code>
    <Message>The Content-MD5 you specified did not match what we received.</Message>
    <ExpectedDigest>VJXmhwy/D/WTNmj8oPUWhA==</ExpectedDigest>
    <CalculatedDigest>1L1i4pARxmfJaEzIV0JVag==</CalculatedDigest>   
    <RequestId>951A188CE57205D4</RequestId>  
    <HostId>FsGh/wbmZDOw7kRk9hGZ078NVvBe1VV7p3U1ac0b8Xp9dakzvVyVrg344xQqvWEy</HostId>
</Error>