client: fix connection-errors being shadowed by API version mismatch errors

Commit e6907243af applied a fix for situations
where the client was configured with API-version negotiation, but did not yet
negotiate a version.

However, the checkVersion() function that was implemented copied the semantics
of cli.NegotiateAPIVersion, which ignored connection failures with the
assumption that connection errors would still surface further down.

However, when using the result of a failed negotiation for NewVersionError,
an API version mismatch error would be produced, masking the actual connection
error.

This patch changes the signature of checkVersion to return unexpected errors,
including failures to connect to the API.

Before this patch:

    docker -H unix:///no/such/socket.sock secret ls
    "secret list" requires API version 1.25, but the Docker daemon API version is 1.24

With this patch applied:

    docker -H unix:///no/such/socket.sock secret ls
    Cannot connect to the Docker daemon at unix:///no/such/socket.sock. Is the docker daemon running?

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/network_create_test.go

@@ -25,6 +25,18 @@ func TestNetworkCreateError(t *testing.T) {
 	assert.Check(t, is.ErrorType(err, errdefs.IsSystem))
 }
 
+// TestNetworkCreateConnectionError verifies that connection errors occurring
+// during API-version negotiation are not shadowed by API-version errors.
+//
+// Regression test for https://github.com/docker/cli/issues/4890
+func TestNetworkCreateConnectionError(t *testing.T) {
+	client, err := NewClientWithOpts(WithAPIVersionNegotiation(), WithHost("tcp://no-such-host.invalid"))
+	assert.NilError(t, err)
+
+	_, err = client.NetworkCreate(context.Background(), "mynetwork", types.NetworkCreate{})
+	assert.Check(t, is.ErrorType(err, IsErrConnectionFailed))
+}
+
 func TestNetworkCreate(t *testing.T) {
 	expectedURL := "/networks/create"