Show proper error message when image upload fails

fixes #994

![selection_006](https://f.cloud.github.com/assets/176576/1368886/28bfaf92-39c6-11e3-9b85-451e84421294.png)
![selection_007](https://f.cloud.github.com/assets/176576/1368887/28c7b20a-39c6-11e3-84b4-e2a2bb163aeb.png)