Fixes #72. Update URL, introduce error when ListenAndServe fail.

[ghost]

1. The URL where the server is running is updated so that it can be clicked from the terminal, in case the terminal supports it.
2. The error is thrown if the port cannot be bound by the tutorial server.

Before:

python -m http.server 8000
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...

go run .
Serving Go+ tutorial at localhost:8000

After

go run .
Serving Go+ tutorial at http://localhost:8000
2024/01/17 11:28:37 Failed to start server at http://localhost:8000.
Error: listen tcp 127.0.0.1:8000: bind: address already in use.
exit status 1