Improve error message when imagemagick is not installed

lib/utils/images.js

@@ -18,7 +18,14 @@ var convert = function(source, dest, options) {
 	if (options.resize)  img = img.resize(options.resize.w, options.resize.h);
 
 	img.noProfile()
-	.write(dest, d.makeNodeResolver());
+	.write(dest, function(err) {
+		if (!err) return d.resolve();
+
+		if (err.code == "ENOENT") {
+			err = new Error("Need to install 'ImageMagick'");
+		}
+		d.reject(err);
+	});
 
 	return d.promise;
 };

lib/utils/page.js

@@ -155,8 +155,9 @@ function convertImages(images, options) {
             return imgUtils.convert(imgin, imgout)
             .then(function() {
                 options.book.log.debug.ok();
-            }, function() {
+            }, function(err) {
                 options.book.log.debug.fail();
+                throw err;
             });
         });
     }, Q());