Fixes angular-ui/ui-utils#204 - improve error message and readme notes about viewport height.

A quick change to improve the error message for missing height elements and some clarification to the README.

So I'm part of the solution and not just an annoying noob. Bin this if it doesn't look useful or amend as you see fit.

![image](https://f.cloud.github.com/assets/959972/2485270/d90a740a-b116-11e3-8b9d-05573b749572.png)