GODRIVER-3361 Improve connection error message.

[qingyang-hu]

GODRIVER-3361

Summary

Improve connection error message.

Background & Motivation

[mongodb-drivers-pr-bot]

API Change Report

No changes found!

[prestonvasquez]

Suggested change

	messages = append(messages, "socket was unexpectedly closed")
	messages = append(messages, "connection closed unexpectedly by the other side")

The ticket suggests "Wrap[ping] io.EOF with an additional error message, like 'connection closed unexpectedly by the other side'". The idea being that we let the user know nothing driver-side is responsible. When the driver closes a connection I would expect something like this: use of closed network connection. Here is a gist with both examples: https://gist.github.com/prestonvasquez/bbab8370950e9c98fa422b6b1ae1069a

[prestonvasquez]

Suggest adding a compile check for ConnectionError, I just noticed we don't do this:

var _ error = ConnectionError{}

[prestonvasquez]

the transformNetworkError function will wrap the error with context.DeadlineExceeded. Should we include that case here for safety?

[prestonvasquez]

We should create integration tests that verify all three branches work as expected. For the io.EOF you could do something like this:

	addr := bootstrapConnections(t, 1, func(nc net.Conn) {
		_ = nc.Close() // Close the connection "server-side" / "other-side"
	})

	p := newPool(
		poolConfig{Address: address.Address(addr.String())},
	)
	defer p.close(context.Background())
	err := p.ready()
	require.NoError(t, err)

	conn, err := p.checkOut(context.Background())
	fmt.Println(err)

	_, err = conn.readWireMessage(context.Background())
	fmt.Println(err) // expect io.EOF

The timeout branches would just require holding a server response > than the timeout. I know we already do this with TestBackgroundRead but that is a secondary check of those tests. We should verify this in a test designed specifically to do som.