Update tests to check returned errors

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

client/request_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/errdefs"
 	"gotest.tools/assert"
 	is "gotest.tools/assert/cmp"
 )
@@ -88,6 +89,9 @@ func TestPlainTextError(t *testing.T) {
 	if err == nil || err.Error() != "Error response from daemon: Server error" {
 		t.Fatalf("expected a Server Error, got %v", err)
 	}
+	if !errdefs.IsSystem(err) {
+		t.Fatalf("expected a Server Error, got %T", err)
+	}
 }
 
 func TestInfiniteError(t *testing.T) {