[drush] xmlsitemap-rebuild should output WARNING instead of ERROR when there are no links rebuildable

Problem/Motivation

When run drush xmlsitemap-rebuild with no rebuildable link types, the drush command output an ERROR (stderr) No link types are rebuildable. [Error].

This kind of message make sense to be an simple WARNING, because there are no errors during the sitemap generation, just a message describing that there are no links to be rebuildable.

This error could interrupt the execution of some CI process (my case)

Proposed resolution

Change "drush_set_error" to "drush_log".

-    return drush_set_error(dt('No link types are rebuildable.'));
+    return drush_log(dt('No link types are rebuildable.'), 'warning');