Add spec that we get a reasonable error message when ImageMagick and GraphicsMagick are both unavailable.  This is an issue with v3.7.0 that was (accidentally?) fixed in commit 2f6f7a1, and I would like it to stay fixed.  The previous error message was "undefined method `size' for nil:NilClass", which is not very helpful, while the current one says "No such file or directory - identify".