Document and test failing to bind a port on a server

[nathanielmanistaatgoogle]

I believe that what will happen is that grpc.Server.add_insecure_port and grpc.Server.add_secure_port return 0 when they fail, but we don't currently document this and I don't recall our having a test for it.

[nathanielmanistaatgoogle]

(gRPC Python should probably raise an exception when failing to bind a port?)

[mehrdada]

I think that is the Python behavior indeed, which is curiously different from C++ ServerBuilder, where the actual binding gets deferred to server initialization, therefore, the integer output argument that gets filled only gets written to later in the server creation step and not immediately after AddPort.

[nathanielmanistaatgoogle]

Closing as a duplicate of issue 11394.