web: log HTTP server errors through the provided slog logger (#461)

• web: log HTTP server errors through the provided slog logger

Serve never set http.Server.ErrorLog, so errors that net/http logs itself, such as “http: TLS handshake error from …: remote error: tls: unknown certificate”, went to the log package’s default logger as plain text. With a JSON logger this produced non-JSON lines.

Set ErrorLog to slog.NewLogLogger(logger.Handler(), slog.LevelError) when the server does not have one yet. ServeMultiple sets it before it starts the Serve goroutines, so the Serve goroutines only read ErrorLog. An ErrorLog set by the caller is kept.