made config saver failures more visible.
made config saver failures more visible.

Previously we ignored saver failures silently. But that means some
issues may've be unnoticed. Added to trace down MB-5110

Change-Id: I6705457a70d309ef006eec9448a6f99f342c39a2
Reviewed-on: http://review.couchbase.org/15050
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,vzasade/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,vzasade/ns_server,membase/ns_server,membase/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,daverigby/ns_server,membase/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,membase/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server