Fix warning about rejected promise without an error

Closes #81

Author: vvasilev-

examples/group-by.md

@@ -18,7 +18,7 @@ var opts = {
 	spritePath: './css/images/',
 	groupBy: function(image) {
 		if (image.url.indexOf('shapes') === -1) {
-			return Promise.reject();
+			return Promise.reject(new Error('Not a shape image.'));
 		}
 
 		return Promise.resolve('shapes');

src/core.js

@@ -110,7 +110,7 @@ export function prepareGroupBy(opts) {
 				return Promise.resolve(`@${image.ratio}x`);
 			}
 
-			return Promise.reject();
+			return Promise.reject(new Error('Not a retina image.'));
 		});
 	}