Fix probe & app's shutdown to please signals.SignalHandlerLoop

Fixes:
```
not enough arguments in call to signals.SignalHandlerLoop
	have ()
	want (logging.Interface, ...signals.SignalReceiver)
```

probe/probe.go

@@ -110,9 +110,10 @@ func (p *Probe) Start() {
 }
 
 // Stop stops the probe
-func (p *Probe) Stop() {
+func (p *Probe) Stop() error {
 	close(p.quit)
 	p.done.Wait()
+	return nil
 }
 
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