MB-5307: crash ebucketmigrator on seeing any exit signal
MB-5307: crash ebucketmigrator on seeing any exit signal

As can be seen in logs of MB-6058 if 'vbucket filter change
transaction' process that's linked to ebucketmigrator fails, we need
to fail bucketmigrator too. Which did not happen previously.

Change-Id: Ib657edfc14cf51af33bb9c88ca7b0831d00b21c1
Reviewed-on: http://review.couchbase.org/19518
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

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