utils.Retry: pass up timeout arguments

If Retry has to fail with RetryTimeout, it might be useful to pass the
Retry argument to RetryTimeout, to help debugging outside the Retry
cycle. We also define a RetryTimeout.RaiseInner() helper, to re-raise an
exception passed to RetryAgain. All served with a side of unit tests.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>