Revisit Browse-By Responses for DSpace 1.8.0

This issue is a placeholder to revisit the decisions made in the 1.7.0 release around DS-640 (Internal System Error when browsing with wrong argument).

For the 1.7.0 Release we decided to do the following:

• For invalid browse types (e.g. /browse?type=NOTVALID), we return a 404 response (and an error page)
• For a missing browse type (e.g. /browse), we return the first browse-by index (by default this is the "browse by date" page)

The Committers decided we should revisit this decision later on to determine it is still applicable and the best decision for all major web crawlers.