Only process EndDevices during add and more verbose errors

If a Coordinator is present in the device list, AddDevicesFromJSON will
fail catastrophically, which shouldn't happen. Therefore, make sure only
EndDevices are considered during add. Also updated the tests to check
for this.

Added a device descriptor for failed adds. This will help with
identifying which device failed (and perhaps, why).

bridge.go

@@ -442,6 +442,12 @@ func (br *Bridge) accessories() []*accessory.A {
 	return acc
 }
 
+func deviceJsonDescriptor(d Device) []byte {
+	d.Definition = nil
+	j, _ := json.Marshal(d)
+	return j
+}
+
 // Creates and calls AddDevice() based on the JSON definitions from zigbee2mqtt/bridge/devices.
 func (br *Bridge) AddDevicesFromJSON(devJson []byte) error {
 	var devices []Device
@@ -458,12 +464,13 @@ func (br *Bridge) AddDevicesFromJSON(devJson []byte) error {
 			if err == ErrDeviceSkipped || err == ErrUnknownDeviceType {
 				continue
 			}
-			return err
+			return fmt.Errorf("createAccessory failed: %+v %s", err, deviceJsonDescriptor(dev))
+
 		}
 
 		err = br.AddDevice(&dev, acc, exp)
 		if err != nil {
-			return err
+			return fmt.Errorf("AddDevice failed: %+v %s", err, deviceJsonDescriptor(dev))
 		}
 	}