Only use the API message when we actually didn't get any raw content

Switches to checking the API response data rather than the stash after
processing through HTML::Restrict.  If HTML::Restrict fails, the lack of
pod may not be related to any API message.
Encourage folks to read the source when a file doesn't have any POD

Previously it rendered as "Error rendering POD for ... - ", with no
explanation.  See #1139.
bin/generate_sitemap.pl needs to be executable.