singleflight: add panicError.Unwrap method

Currently when singleflight recovers from a panic, it wraps it with the private
error type panicError. This change adds an `Unwrap` method to panicError to
allow wrapped errors to be returned.

Updates golang/go#62511

Change-Id: Ia510ad7d5881207ef71f9eb89c1766835af19b6b
Reviewed-on: https://go-review.googlesource.com/c/sync/+/526171
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

singleflight/singleflight.go

@@ -31,6 +31,15 @@ func (p *panicError) Error() string {
 	return fmt.Sprintf("%v\n\n%s", p.value, p.stack)
 }
 
+func (p *panicError) Unwrap() error {
+	err, ok := p.value.(error)
+	if !ok {
+		return nil
+	}
+
+	return err
+}
+
 func newPanicError(v interface{}) error {
 	stack := debug.Stack()