use exit status of compare instead of STDOUT

Version 6.8.8-9 of ImageMagick's compare returns 2 on error, 0 if the images are similar, and 1 if they are dissimilar. Rather than using the output of compare, this changes the code to use the actual return code.

When using compare, the actual output of the command is "0 @ 0,0" if the images are similar. I'm presuming, based on the previous code, that "0" was output to STDOUT (or STDERR) if the images were similar.

A lot more tests pass on my system after this changed compared to before the change. The tests that still fail are expecting a number in the diff while compare is now instead returning "compare: images too dissimilar `<path to image' @ error/compare.c/CompareImageCommand/976." which… I'm not sure how to adjust. The behavior is proper, but the non-matching image output no longer matches what is expected by the tests.