Display more explicit page when link is not available

[benoit74]

When something is not available for offline browsing, the user must be displayed with clearer information.

We must make the distinction between cases where:

• the data is not yet available for offline browsing due to scrapper limitations
• the data will never be available due to its dynamic nature (forms, forum, ...)
• the data is external to the website and hence not scrapped
• the data is not available in the current ZIM version due to configuration (ZIM is only a selection)

[benoit74]