Erratum: The printed version of the paper contains a minor error in
the beginning of the proof of Theorem 2.4.  This is corrected in the
version provided in this web-site. 

Bounding $\Vert G_{ns}^1 + G_{ns}^2 \Vert$ in the proof of Theorem
2.4, we used the incorrect inequality: 
$$ 
k \sum_{j=s+1}^n
t_{n-j+1}^{-1-\delta} \le C t_{n-s}^{-\delta}.  
$$  
Nevertheless, although the proof is incorrect, the estimate itself
remains valid.  The correct argument goes as follows.

For this purpose we use (1.9) and Lemma~2.1 to get, similarly to the
proof of Theorem~2.3,
$$
\|G^1_{ns}+G^2_{ns}\|
\le C\omega_{ns}\Big( 1+\|E_k^{n-s}\|
    +k\sum_{j=s+1}^n t_{n-j+1}\|AE_k^{n-j+1}\| \Big)
\le CM_\delta t_{n-s}^{-\delta}\omega_{ns}, 
$$