Client errors are quite often wrapped in context. When checking a
specific error, it is required not to lose the ability to determine it,
regardless of the attached context. In previous implementation `IsErr*`
functions didn't support wrapped errors.
Make `IsErr*` functions to preliminarily unwrap `error` argument before
type assertion. Use `errors.Unwrap` for this instead of `errors.As`
since the latter has the overhead of filling in an error.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
