Wrap internal typed errors to help the users better handle errors

[lezh]

This library was created before Go 1.13 where error wrapping was introduced. So, it did not wrap errors, which made error handling at the caller side a bit tricky.

For instance, when opening an unknown bucket, the HTTP 404 error was replaced by a custom "Unknown bucket" error.
https://github.com/jacobsa/gcloud/blob/master/gcs/conn.go#L191-L192

		case http.StatusNotFound:
			err = fmt.Errorf("Unknown bucket %q", b.Name())

This error is hard to be handled by the caller. However, if we wrap the internal http error like this:

		case http.StatusNotFound:
			err = fmt.Errorf("Unknown bucket %q: %w", b.Name(), err)

On the user side, such error can be casted back to an 404, and properly handled:

	if errors.As(err, &apiErr) {
		switch apiErr.Code {
		case http.StatusNotFound:
			return syscall.EINVAL
		}
	}