config: Improve error message when test socket creation fails

When Gitaly starts up, it will verify that it can indeed create Unix sockets in its runtime directory. This check is required because Unix systems have strict limits on Unix socket path lengths due to struct sockaddr_un's sun_path is a capped character array. On macOS this limit is at 104 characters, while Linux has a limit of 108 characters, both of which are easy to exceed.

But while we do have these sanity checks in place, the error message we return is really hard for an administrator to interpret because all it talks about is an invalid argument passed to bind(3P). So ultimately, the administrator likely won't make the connection between the runtime directory being too long and an invalid argument.

Improve this error message by catching EINVAL returned by net.Listen() and providing a readable error message that explicitly tells the admin that the issue is most likely that the path is too long and that the runtime directory should be changed.

Part of gitlab-development-kit#1480 (closed).