Fix: Improve no-extra-parens error message

When I encountered this error, it was not unclear to me what the
problem was or what alternative would be preferred.

The input was:

```js
return ( { foo: 1, bar: 2 }[ key ] || 0 );
```

Receiving the following in Atom's footer:

```
eslint (0|4), Gratuitous parentheses around expression., Line 165, Column 17
```

lib/rules/no-extra-parens.js

@@ -58,7 +58,7 @@ module.exports = {
         },
 
         messages: {
-            unexpected: "Gratuitous parentheses around expression."
+            unexpected: "Unexpected parentheses around expression."
         }
     },