client: NegotiateAPIVersion: do not ignore (connection) errors from Ping

NegotiateAPIVersion was ignoring errors returned by Ping. The intent here
was to handle API responses from a daemon that may be in an unhealthy state,
however this case is already handled by Ping itself.

Ping only returns an error when either failing to connect to the API (daemon
not running or permissions errors), or when failing to parse the API response.

Neither of those should be ignored in this code, or considered a successful
"ping", so update the code to return

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

client/client_test.go

@@ -354,6 +354,19 @@ func TestNegotiateAPVersionOverride(t *testing.T) {
 	assert.Equal(t, client.ClientVersion(), expected)
 }
 
+// TestNegotiateAPVersionConnectionFailure asserts that we do not modify the
+// API version when failing to connect.
+func TestNegotiateAPVersionConnectionFailure(t *testing.T) {
+	const expected = "9.99"
+
+	client, err := NewClientWithOpts(WithHost("unix:///no-such-socket"))
+	assert.NilError(t, err)
+
+	client.version = expected
+	client.NegotiateAPIVersion(context.Background())
+	assert.Equal(t, client.ClientVersion(), expected)
+}
+
 func TestNegotiateAPIVersionAutomatic(t *testing.T) {
 	var pingVersion string
 	httpClient := newMockClient(func(req *http.Request) (*http.Response, error) {