Fix detection of "Nothing to compile" situation

`compass compile` errors weren't failing the task.  I'm using Compass 0.12.2 and testing on both Mac and Linux.

The logic to detect the "Nothing to compile" situation seems wrong - at least for 0.12.2.  If `result.stderr` was truthy for any reason (which is the case for typical compile errors), the task would succeed.  Also, "Nothing to compile" appears in stderr, not stdout, on the versions/platforms I'm testing.

The build succeeds after my changes.

![Screen Shot 2013-03-28 at 10 57 16 AM](https://f.cloud.github.com/assets/1527302/314657/f768b674-97d0-11e2-9ce4-0b62bacff3ae.png)