cleanup: exceptions should derive from Exception
cleanup: exceptions should derive from Exception

Reviewed-by: amishchenko

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