Refactor errors functions

[qualand]

It would be good to move the code from each of the cases of this switch statement to their own functions. The code would be more readable that way.

Originally posted by @jmaack24 in #75 (comment)

[qualand]

The two functions in tracing_errors.cpp are basically the same. Refactor to avoid code duplication.

NOTES:
SurfaceNormalErrors() Errors()

• uses slope error - uses specularity error
• applies to surface normal (input) - applies to ray direction (input)
• no diffuse option - has diffuse option
  - has an dot product check (goto) for surface reflection (requires DFXYZ)
  - handles sun shape errors

Plan: Break up surface and sun shape error handling into separate functions

• remove the special case of diffuse surfaces before Errors()
• Reduce the random number calls. I.e., sample theta directly rather than thetax and thetay -> this will break tests because the number of RNG calls will change.