Add a test for aiohttp.ClientOSError exceptions in parsers

[jacobtomlinson]

In both the api.ai and LUIS parsers there is a try ... except block for an aiohttp.ClientOSError when contacting the API.

These lines are not covered in the tests. This needs adding.

[FabioRosado]

While browsing stackoverflow and checking the wit.ai test cases I came up with this:

    async def test_parse_witai_clientOSError(self):
        with OpsDroid() as opsdroid:
            opsdroid.config['parsers'] = [
                {'name': 'witai', 'access-token': 'test'}
            ]
            mock_skill = amock.CoroutineMock()
            match_witai('get_weather')(mock_skill)

            with amock.patch.object(witai, 'call_witai') as mocked_call_witai:
                mocked_call_witai.raiseError.ClientOSError = ClientOSError()

            self.assertTrue(ClientOSError)

This simply raises the ClientOSError and assets it to true. Would this be something worth using for this issue or I should put more thought into it?