Prettyprint stacktrace when provisioning fails

Before:
![ec2-before](https://f.cloud.github.com/assets/206841/730549/8c61c00c-e256-11e2-99f5-17e13f05fc8d.png)
After:
![ec2-after](https://f.cloud.github.com/assets/206841/730550/99313010-e256-11e2-8c4f-8f2c403d63af.png)