Improve error reporting for glacier uploads

We now get more detailed error logs such as:

```
ERROR Exception caught uploading part number 0 for vault vault_name, attempt: (2 / 3), filename: /var/folders/rb/i, exception: <type 'exceptions.Exception'>, msg: foo
```

I also had to fix the num_retries to be more sensical.  The
"attempts" part of the above log message uses num_retries + 1 for the
total number of attempts which is more intuitive.