Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

src/display/font_loader.js

@@ -376,7 +376,7 @@ var FontFaceObject = (function FontFaceObjectClosure() {
           this.options.fontRegistry.registerFont(this);
         }
         return nativeFontFace;
-      } else {
+      } else { // eslint-disable-line no-else-return
         throw new Error('Not implemented: createNativeFontFace');
       }
     },