Fix error message displaying when uploading a screenshot (bug 928287)

https://bugzilla.mozilla.org/show_bug.cgi?id=928287

The heart of the problem was the missing `<div class="edit-previews-text">` which is used by the js to display the errors, but I had to modify the CSS too to make everything fit correctly with the last style modifications. Also a drive-by fix to stop us from trying to load  "/developers/app/<app>/edit/false" by checking `fileURL` before using it as a background-image.

Post-fix screenshot:
![fix-screenshot-error](https://f.cloud.github.com/assets/187006/1360192/48424a54-37f1-11e3-84bd-7e20425366ad.png)