Add spec that we get a reasonable error message when ImageMagick is not found

As the commit message says, 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".

The weird error message was a problem for me on a server for a few days until I realized that my PATH was set incorrectly.