Ticket created following this SO question: http://stackoverflow.com/questions/41940996/initial-indexation-in-hibernate-search-with-elasticsearch-crashes/41944640
... and a discussion on HipChat.

We should:

1. Rephrase the sentence in the docs about the delete-by-query plugin to mention this is about the MassIndexer.
2. Promote the "ES config" section to a level 3 section and move it before the HS configuration section (which would have to be renamed "Hibernate Search configuration")
3. If possible, adapt the "purge" code to send a specific exception when we detect that the plugin isn't available (404 error?). Not sure if we can distinguish that kind of error from a missing index, though.