Fix custom prose namespacing to fit with documentation

I was trying to use the custom prose namespace feature and was getting class not found error for a thrown exception, additionally the error seems to not match the documentation here http://datasift.github.io/storyplayer/configuration/prose-namespaces.html. 

This adds the missing exception and fixes the config check so that it checks that prose->namespaces is an array not prose.

Wasn't too sure with the exception the naming style used, InvalidConfig seems to be a lot more generic than the other exceptions, I kept it as was for now but can change to something more specific if needed.