pkg/ifaceprobe: note error only when res.Err is not null

Seeing iface-probe run exits abnormally:

```
[FATAL] interface probing failed: failed to execute prog: %!w(<nil>) (Crashed)
r0 = openat(0xffffffffffffff9c, &(0x7f0000000080)='/sys/devices/pci0000:00/0000:00:01.1/
	ata1/host0/target0:0:0/0:0:0:0/block/sda/queue/scheduler', 0x1, 0x0)
write(r0, &(0x7f0000000100)=' ', 0x1)

exit status 1
```

The node to have this `%!w(nil>) (Crashed)` is random, but
continue when res.Err is null bypass the problem.

Author: jiangenj

pkg/ifaceprobe/ifaceprobe.go

@@ -217,8 +217,10 @@ func (pr *prober) submitFile(file string) {
 		for _, req := range reqs {
 			res := req.Wait(pr.ctx)
 			if res.Status != queue.Success {
-				pr.noteError(fmt.Errorf("failed to execute prog: %w (%v)\n%s\n%s",
-					res.Err, res.Status, req.Prog.Serialize(), res.Output))
+				if res.Err != nil {
+					pr.noteError(fmt.Errorf("failed to execute prog: %w (%v)\n%s\n%s",
+						res.Err, res.Status, req.Prog.Serialize(), res.Output))
+				}
 				continue
 			}
 			desc.results = append(desc.results, res)