cmdlib: simplify some rpc error handling cases
cmdlib: simplify some rpc error handling cases

By using the RemoteFailMsg() or the payload field of RpcResult, we can
simplify a few functions in cmdlib.

Reviewed-by: ultrotter

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