MB-6490: don't ignore failures in wait_checkpoint_persisted_many
MB-6490: don't ignore failures in wait_checkpoint_persisted_many

Previously we lacked any error handling there. Causing us to ignore
crashes, sadly.

Change-Id: I87da25c4611c810611dcc510b748b8521455c138
Reviewed-on: http://review.couchbase.org/21552
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

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