In error, the first letter is low-case letter

Signed-off-by: YuPengZTE <yu.peng36@zte.com.cn>

daemon/graphdriver/devmapper/deviceset.go

@@ -1785,7 +1785,7 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 			hasData := devices.hasImage("data")
 
 			if !doInit && !hasData {
-				return errors.New("Loopback data file not found")
+				return errors.New("loopback data file not found")
 			}
 
 			if !hasData {
@@ -1818,7 +1818,7 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 			hasMetadata := devices.hasImage("metadata")
 
 			if !doInit && !hasMetadata {
-				return errors.New("Loopback metadata file not found")
+				return errors.New("loopback metadata file not found")
 			}
 
 			if !hasMetadata {