Added a more descriptive error message when "category" is omitted

Whilst testing Theo, I was getting the following error when running gulp...

```
Cannot call method 'toLowerCase' of undefined
```

I had to dig through the source to find out it was because i'd forgotten to add a "category" to a style property. I felt a more descriptive error would have saved me a time troubleshooting the issue, so made this PR. 

Example output:
```
COLOR_INDIGO_DARK did not contain a "category" property
```