MB-8420: Change invalid view name error to "not found"

If you query with a view name that doesn't exist in a design
document and error with HTTP status code 500 was returned.
This commit changes it to a not_found error with HTTP status
code 404.

Change-Id: Ia2df3e7663a84eb0c8de97a609301de63e6e8e72
Reviewed-on: http://review.couchbase.org/26973
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>