Remove useless error message

All errors are already logged from the called functions: readSMARTctl,
resultCodeIsOk, jsonIsOk. Logging twice due to the same error is useless
and the "smartctl returned bad data for device" message is confusing
since that condition may happen when a device is in a low-power mode.

Signed-off-by: Jakub Klinkovský <j.l.k@gmx.com>

main.go

@@ -49,12 +49,11 @@ func (i SMARTctlManagerCollector) Describe(ch chan<- *prometheus.Desc) {
 func (i SMARTctlManagerCollector) Collect(ch chan<- prometheus.Metric) {
 	info := NewSMARTctlInfo(ch)
 	for _, device := range i.Devices {
-		if json, err := readData(i.logger, device); err == nil {
+		json := readData(i.logger, device)
+		if json.Exists() {
 			info.SetJSON(json)
 			smart := NewSMARTctl(i.logger, json, ch)
 			smart.Collect()
-		} else {
-			level.Error(i.logger).Log("msg", "Error collecting SMART data", "err", err.Error())
 		}
 	}
 	info.Collect()

readjson.go

@@ -89,9 +89,9 @@ func readSMARTctlDevices(logger log.Logger) gjson.Result {
 }
 
 // Select json source and parse
-func readData(logger log.Logger, device string) (gjson.Result, error) {
+func readData(logger log.Logger, device string) gjson.Result {
 	if *smartctlFakeData {
-		return readFakeSMARTctl(logger, device), nil
+		return readFakeSMARTctl(logger, device)
 	}
 
 	cacheValue, cacheOk := jsonCache.Load(device)
@@ -103,11 +103,11 @@ func readData(logger log.Logger, device string) (gjson.Result, error) {
 			if !found {
 				level.Warn(logger).Log("msg", "device not found", "device", device)
 			}
-			return j.(JSONCache).JSON, nil
+			return j.(JSONCache).JSON
 		}
-		return gjson.Parse("{}"), fmt.Errorf("smartctl returned bad data for device %s", device)
+		return gjson.Result{}
 	}
-	return cacheValue.(JSONCache).JSON, nil
+	return cacheValue.(JSONCache).JSON
 }
 
 // Parse smartctl return code