Show a proper error message for protected domain names

lib/alias.js

@@ -200,6 +200,12 @@ export default class Alias extends Now {
           return bail(err);
         }
 
+        if ('forbidden' === code) {
+          const err = new Error('The domain you are trying to use as an alias is already in use by a different account.');
+          err.userError = true;
+          return bail(err);
+        }
+
         return bail(new Error('Authorization error'));
       }