Add scene names to operator and token error messages.

[sgranade]

Minor patch to add scene names to errors that previously were only showing line numbers. This impacts one token and a number of operators, and the underlying utils functions (num() and bool()).

[dfabulich]

This breaks a bunch of unit tests. (ant unittest will run them.)

[sgranade]

Whoops, sure enough! I didn't handle null values of scene objects properly. I fixed that, and added unit tests for the error messages as well.

[dfabulich]

Thanks!