Verify: fix crash when a node is down
Verify: fix crash when a node is down

Currently if ganeti-noded doesn't respond on a node gnt-cluster verify will die
when verifying primary instances for that node. Fix this by just emitting an
error message if no information about running instances is returned from the
node.

Reviewed-by: iustinp



grnet/snf-ganeti,mbakke/ganeti,yiannist/ganeti,yiannist/ganeti,apyrgio/ganeti,dimara/ganeti,ganeti/ganeti,andir/ganeti,dimara/ganeti,apyrgio/snf-ganeti,apyrgio/snf-ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,leshchevds/ganeti,mbakke/ganeti,andir/ganeti,grnet/snf-ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,leshchevds/ganeti,apyrgio/ganeti,andir/ganeti,onponomarev/ganeti,leshchevds/ganeti,bitemyapp/ganeti,mbakke/ganeti,yiannist/ganeti,ganeti/ganeti