Improved error message when generator is not found

As discussed in #196, this replaces the use of "namespace" with generator and provides instructions how to find and install a missing generator.

![Screenshot from 2013-03-26 18:17:31](https://f.cloud.github.com/assets/9906/304307/75e77efc-9639-11e2-9860-ec907ea3ca27.png)