Improve error msg when port is not available

[dceleda]

Very often when a default port (30303) is not available, for example, because another instance is running, we got a SocketException saying that "address already in use". We could improve that message for a user by adding smth like: "Please make sure that you're not running other applications on port {port}. Check your Network.DiscoveryPort and Network.P2PPort in configuration"

[glunkad]

Hey @dceleda , I'm interested in contributing to this issue, so before I start working on it, would you mind sparing your time explaining what the issue is about and pointing me to some resources to get started.

[dceleda]

@rubo could you help @9gl ?

[glunkad]

@dceleda or @rubo Could you point me to the file to get started?

[rubo]

@9gl Please see RlpxHost.cs