Show errors from ImageMagick

[danvk]

The "differing pixels" feature wasn't working on my laptop recently. It just silently failed. When I print the output of compare, I get an error:

compare: unable to load module `/usr/local/Cellar/imagemagick/6.9.2-4/lib/ImageMagick//modules-Q16/coders/png.la': file not found @ error/module.c/OpenModule/1300.
compare: no decode delegate for this image format `PNG' @ error/constitute.c/ReadImage/501.
compare: unable to load module `/usr/local/Cellar/imagemagick/6.9.2-4/lib/ImageMagick//modules-Q16/coders/png.la': file not found @ error/module.c/OpenModule/1300.
compare: no decode delegate for this image format `PNG' @ error/constitute.c/ReadImage/501.

This should be surfaced in some way.