client-go: properly return error in fake discovery (#79612)

Fake discovery should return an error if an error-returning reactor was prepended.

This is relevant e.g. for unit tests which test a function which relies
on discovery to check if an API Server is available.

Matches existing behavior of other fake types, for example fake_namespace.

Adds unit test coverage as well.

Kubernetes-commit: b47748103a53c0fe5ba1ddc950662ffc9bd9ed0f

discovery/fake/discovery_test.go

@@ -17,11 +17,14 @@ limitations under the License.
 package fake_test
 
 import (
+	"errors"
 	"testing"
 
+	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/version"
 	fakediscovery "k8s.io/client-go/discovery/fake"
 	fakeclientset "k8s.io/client-go/kubernetes/fake"
+	kubetesting "k8s.io/client-go/testing"
 )
 
 func TestFakingServerVersion(t *testing.T) {
@@ -44,3 +47,19 @@ func TestFakingServerVersion(t *testing.T) {
 		t.Fatalf("unexpected faked discovery return value: %q", sv.GitCommit)
 	}
 }
+
+func TestFakingServerVersionWithError(t *testing.T) {
+	expectedError := errors.New("an error occurred")
+	fakeClient := fakeclientset.NewSimpleClientset()
+	fakeClient.Discovery().(*fakediscovery.FakeDiscovery).PrependReactor("*", "*", func(action kubetesting.Action) (handled bool, ret runtime.Object, err error) {
+		return true, nil, expectedError
+	})
+
+	_, err := fakeClient.Discovery().ServerVersion()
+	if err == nil {
+		t.Fatal("ServerVersion should return error, returned nil instead")
+	}
+	if err != expectedError {
+		t.Fatal("ServerVersion should return expected error, returned different error instead")
+	}
+}