Optional `imagemagick` part 2:

if (!im) {
    throw new Error('Invalid Configuration\n\n' +
        'fsImage fields (' + list.key + '.' + path + ') require gm v1.14.2 to be installed.\n\n' + 
            'See http://keystonejs.com/guide/fields/#fsimage for more information.\n');
}

... so if they don't have the package, **and** try and use an fsimage field, it throws with a pointer to the docs.