i2c: return error correctly, increase test coverage for l3gd20h

Signed-off-by: deadprogram <ron@hybridgroup.com>

drivers/i2c/l3gd20h_driver.go

@@ -133,7 +133,7 @@ func (d *L3GD20HDriver) Halt() (err error) {
 // XYZ returns the current change in degrees per second, for the 3 axis.
 func (d *L3GD20HDriver) XYZ() (x float32, y float32, z float32, err error) {
 	if _, err = d.connection.Write([]byte{l3gd20hRegisterOutXLSB}); err != nil {
-		return 0, 0, 0, nil
+		return 0, 0, 0, err
 	}
 	measurements := make([]byte, 6)
 	if _, err = d.connection.Read(measurements); err != nil {

drivers/i2c/l3gd20h_driver_test.go

@@ -102,6 +102,17 @@ func TestL3GD20HDriverMeasurementError(t *testing.T) {
 	gobottest.Assert(t, err, errors.New("read error"))
 }
 
+func TestL3GD20HDriverMeasurementWriteError(t *testing.T) {
+	d, adaptor := initTestL3GD20HDriverWithStubbedAdaptor()
+	d.Start()
+	adaptor.i2cWriteImpl = func(b []byte) (int, error) {
+		return 0, errors.New("write error")
+	}
+
+	_, _, _, err := d.XYZ()
+	gobottest.Assert(t, err, errors.New("write error"))
+}
+
 func TestL3GD20HDriverSetName(t *testing.T) {
 	d := initTestL3GD20HDriver()
 	d.SetName("TESTME")