write_req raise KeyError when connection is disconnected

[juhhov]

KeyError is raised when connection is disconnected just before attempting to write at

pc-ble-driver-py/pc_ble_driver_py/ble_adapter.py

Line 500 in a6b36d3

attr_handle = self.db_conns[conn_handle].get_char_value_handle(uuid)

. More specific error should be raised.