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.

Change-Id: Id680d642cb0079461429f3910343c6dbe8d488ef