Avoiding Async exception on serial port failure

[Ircama]

Fix obd.Async() exception when failing to open the serial device.

Fixed exception: AttributeError: 'Async' object has no attribute '_Async__thread'

Fixes https://github.com/brendan-w/python-OBD/issues/67

Testing program:

import obd
import sys
connection = obd.Async(sys.argv[1])

Before the patch:

• No exception: python3 test-async.py /dev/existing-device

• Exception: python3 test-async.py /dev/unexisting-device AttributeError: 'Async' object has no attribute '_Async__thread'

After the patch:

• python3 test-async.py /dev/existing-device no exception

• python3 test-async.py /dev/unexisting-device [obd.elm327] [Errno 2] could not open port /dev/unexisting-device: [Errno 2] No such file or directory: '/dev/unexisting-device' [obd.obd] Cannot load commands: No connection to car