Fix for model shader optimize (linux)

This fixes a linux specific problem where the WebGL shader optimizer is dropping an attribute from the shaders.

**Note:** scene.rethrowRenderErrors is now set to true by default for all specs.  It was required to get this spec working and seemed like the sort of thing that should be the default for specs.  The only spec that started failing due to this change is in Specs/Scene/SceneSpec.js which has been updated accordingly.
Perhaps a cleaner approach is to avoid this `if` statement and do the `for` loop over `programAttributeLocations` since we know these are active attributes in the shader program.

In either case, let's put a comment here so we don't change it later my accident.