feat(storage): returned wrapped error for timeouts

This piggybacks on googleapis#4797 to allow storage to expose wrapped
service errors when a call retries without success until timeout
or cancellation.

I also updated all checks for context sentinel errors in storage
to use xerrors.Is instead of strict equality. Users of this
package should do the same. I'll update the documentation on
errors from this package in a subsequent PR.

We will have to bump the dependency on the root module before
merging this change I believe.

Fixes googleapis#4197

storage/integration_test.go

@@ -3206,14 +3206,12 @@ func TestIntegration_CancelWrite(t *testing.T) {
 	cancel()
 	// The next Write should return context.Canceled.
 	_, err = w.Write(buf)
-	// TODO: Once we drop support for Go versions < 1.13, use errors.Is() to
-	// check for context cancellation instead.
-	if err != context.Canceled && !strings.Contains(err.Error(), "context canceled") {
+	if !xerrors.Is(err, context.Canceled) {
 		t.Fatalf("got %v, wanted context.Canceled", err)
 	}
 	// The Close should too.
 	err = w.Close()
-	if err != context.Canceled && !strings.Contains(err.Error(), "context canceled") {
+	if !xerrors.Is(err, context.Canceled) {
 		t.Fatalf("got %v, wanted context.Canceled", err)
 	}
 }
@@ -3865,7 +3863,7 @@ func TestIntegration_ReaderCancel(t *testing.T) {
 		buf := make([]byte, 1000)
 		_, readErr = r.Read(buf)
 		if readErr != nil {
-			if readErr == context.Canceled {
+			if xerrors.Is(readErr, context.Canceled) {
 				return
 			}
 			break

storage/invoke.go

@@ -36,7 +36,7 @@ func runWithRetry(ctx context.Context, call func() error) error {
 			return true, nil
 		}
 		if shouldRetry(err) {
-			return false, nil
+			return false, err
 		}
 		return true, err
 	})

storage/writer.go

@@ -24,6 +24,7 @@ import (
 	"unicode/utf8"
 
 	"github.com/golang/protobuf/proto"
+	"golang.org/x/xerrors"
 	"google.golang.org/api/googleapi"
 	raw "google.golang.org/api/storage/v1"
 	storagepb "google.golang.org/genproto/googleapis/storage/v2"
@@ -217,7 +218,7 @@ func (w *Writer) Write(p []byte) (n int, err error) {
 		// Preserve existing functionality that when context is canceled, Write will return
 		// context.Canceled instead of "io: read/write on closed pipe". This hides the
 		// pipe implementation detail from users and makes Write seem as though it's an RPC.
-		if werr == context.Canceled || werr == context.DeadlineExceeded {
+		if xerrors.Is(werr, context.Canceled) || xerrors.Is(werr, context.DeadlineExceeded){
 			return n, werr
 		}
 	}