Removing a bunch of fluff from exceptions.

Deriving directly from std::runtime_error instead of std::exception.
Can now visualize the normals of imported assets.

The asset's normals are correct.
Corrected lighting calculations.

Now uLightDirWorld represents the actual direction of the light, instead
of its inverse.
Merge branch 'master' of https://github.com/SCIInstitute/spire