Undefined variable in Newspaper template when page JP2 missing

If you are viewing a newspaper page that is missing its JP2 datastream, you get the following warning:

Notice: Undefined variable: content in include() (line 24 of /var/www/drupal/sites/all/modules/islandora_solution_pack_newspaper/theme/islandora-newspaper-page.tpl.php).

We should check for the content at that point. And if there is no content, either we could avoid the warning by skipping the rest of the stuff associated with $content, or (better approach) we should log an error like "JP2 derivative missing for this:pid".