Fix error reporting for unknown pluggable UI REST prefix
Fix error reporting for unknown pluggable UI REST prefix

Change-Id: I358077bde454c51af58cba01b06e083d9380098f
Reviewed-on: http://review.couchbase.org/58753
Tested-by: Anders Nygren <4f8bff4ce020de40cfee5db6ca7fd51acc06e5c4@couchbase.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server