Asyncio server will set serving=False if it fails to start

[couling]

Asyncio server should notify that it is NOT serving if the server fails to start.

At this time, the server has no server.start_serving() method only a server.serve_forever() method. This means the only way to detect if the server is ready to receive connections await server.serving() with defer_start=False.

This is particularly useful when using the modbus server in a unit test.

import asyncio

async def use_server_for_test(context):
    server = ModbusTcpServer(context)
    server_task = asyncio.get_running_loop().create_task(server.serve_forever())
    try:
        assert await server.serving
        # Test a client by connecting to this server.
        test_my_client(server.server.sockets[0].getsockname())
    finally:
        server_task.cancel()

At the moment the above code would just hang and I can see no other future that will return both when the server starts and if it fails to start.

This PR then sets the serving to False if the server fails to startup.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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0.0% Duplication

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
1.7% Duplication

[janiversen]

This code is merged into dev.