device: fix incorrect uses of MultiError and guard against misuse

device/alsa/alsa.go

@@ -177,7 +177,7 @@ func (d *ALSA) Setup(c config.Config) error {
 	d.mode = paused
 	go d.input()
 
-	if errs != nil {
+	if len(errs) != 0 {
 		return errs
 	}
 	return nil

device/device.go

@@ -68,5 +68,8 @@ type AVDevice interface {
 type MultiError []error
 
 func (me MultiError) Error() string {
+	if len(me) == 0 {
+		panic("device: invalid use of MultiError")
+	}
 	return fmt.Sprintf("%v", []error(me))
 }

device/geovision/geovision.go

@@ -203,8 +203,11 @@ func (g *GeoVision) Set(c avconfig.Config) error {
 	const setupDelay = 5 * time.Second
 	time.Sleep(setupDelay)
 
+	if len(errs) != 0 {
 		return errs
 	}
+	return nil
+}
 
 // Start uses an RTSP client to communicate with the GeoVision RTSP server and
 // request a stream that is then received by an RTP client, from which packets

device/raspistill/raspistill.go

@@ -135,5 +135,8 @@ func (r *Raspistill) Set(c config.Config) error {
 	}
 
 	r.cfg = c
+	if len(errs) != 0 {
 		return errs
 	}
+	return nil
+}

device/raspivid/raspivid.go

@@ -239,8 +239,11 @@ func (r *Raspivid) Set(c config.Config) error {
 	}
 
 	r.cfg = c
+	if len(errs) != 0 {
 		return errs
 	}
+	return nil
+}
 
 func goodAWBGains(g string) bool {
 	parts := strings.Split(g, ",")

device/webcam/webcam.go

@@ -115,8 +115,11 @@ func (w *Webcam) Set(c config.Config) error {
 		c.Bitrate = defaultBitrate
 	}
 	w.cfg = c
+	if len(errs) != 0 {
 		return errs
 	}
+	return nil
+}
 
 // Start will build the required arguments for ffmpeg and then execute the
 // command, piping video output where we can read using the Read method.