Major bug: Removing a bluetooth device causes tinyb crash

[vkolotov]

When a connected device (with enabled characteristic change notification) gets removed through an external tool (bluetoothctl or similar) then tinyb crashes and becomes unresponsive.

Steps to reproduce:

1. BluetoothDevice.connect()
2. BluetoothGattCharacteristic.enableValueNotifications(...)
3. Remove device (e.g. bluetoothctl# remove XX:XX:XX:XX:XX)
4. BluetoothGattCharacteristic.disableValueNotifications() (possibly other methods as well)

Results in:

tinyb.BluetoothException: GDBus.Error:org.freedesktop.DBus.Error.UnknownObject: Method "StopNotify" with signature "" on interface "org.bluez.GattCharacteristic1" doesn't exist

Also, there is not any way to detect whether a device is removed, e.g. it is not possible to have a condition before BluetoothGattCharacteristic.disableValueNotifications method call.