Enforcing compulsory parameters

I've made it so that not specifying $module or $version in the sass mixin will throw an error (see https://github.com/Financial-Times/o-assets/commit/cf74c7c677279090056d51dfc8a54208c3ba79bc also). The error won't be particularly informative as it is though.

@triblondon Please review and let me know:

1. If you can see any problem with throwing an error here
1. If you'd like me to e.g. set default values to `__novalue__', then I can do string matching on these and throw more meaningful errors