Fix some errors in errorCalculator.rs.

Bug: http://b/26987366

This code was accidentally casting a uchar4 to a single integer, which
the latest llvm-rs-cc will flag as an error. The comparison was also
incorrectly using a single integer comparison, instead of looking at all
4 uchar vector components.

(cherry picked from commit 74be0c9b)

Change-Id: Ic3727e234ffc87571ccacab19b2a4efb27ac0f20