add error highlight/underlines for shape generators

this closes #44

this is a first pass, but since so much has changed with #44 it's time we bail on the monster ticket and track further work in more discrete chunks.

![screen shot 2014-11-11 at 2 51 59 am](https://cloud.githubusercontent.com/assets/46673/4991852/ee02924e-694d-11e4-9f68-0810ea6e4754.png)