Make better error reporting for client's call to rados.Rados

This is a quick fix to improve the error reporting when starting the Ceph client. Currently, when there's a problem, it prints out this
```text
Error initializing cluster client: Error
```
This makes it difficult to figure out the actual problem. The advice to use repr comes from http://stackoverflow.com/questions/4308182/getting-the-exception-value-in-python

Signed-off-by: Derrick Schneider <derrick.schneider@gmail.com>