Make test output more verbose if Provider is not available for live tests.

I had some time to manage that 

Live tests are passing in case when service is unavailable. For instance you've put wrong key name into `.fog` file and when you ran `bundle exec rake live[<provider>]` tests always pass. The only thing you can see is a warning 
```
Skipping tests for <provider> due to lacking credentials 
(add some to '.../fog/tests/.fog' to run them)
``` 
There are ~30 similar lines there and I'm not sure every one are reading them. I've spent some time to understand what is wrong and I think this situation needs more verbose output.