Rewrote TestSceneCache to use parallel_reduce.

This fixes an issue where task.errors() was not returning a meaningful result. Now it returns the sum of all errors on all threads. I verified that without the fixes from 030e7c936de091e22564acec4bbffcb222154286 the test still gets the seg fault, and with those changes it passes.
Fixing seg fault when attempting to read a non-existant attribute several times.

This issue was introduced by 030e7c936de091e22564acec4bbffcb222154286
Merge pull request #277 from andrewkaufman/rootAttrCrash

 Fixing seg fault when attempting to read a non-existant attribute several times