title: Add or remove woocommerce error messages
body: <p>i am developing this shipping cost generator for TNT as a WordPress woo-commerce extention.the thing is if there is an error i need to show the error and restrict user from checkout.I used following method to do that.
  The problem is how i can remove this notice when the user entered the correct data and manage the thing done.</p>
  
  <pre><code>if (!empty($xml-&gt;ratedTransitTimeResponse-&gt;ratedProducts-&gt;ratedProduct[0]-&gt;quote-&gt;price)) {
      $cost+= (float)$xml-&gt;ratedTransitTimeResponse-&gt;ratedProducts-&gt;ratedProduct[0]-&gt;quote-&gt;price;
  }
  else wc_add_wp_error_notices(new WP_Error(1, 'Shipping cost error'));
  </code></pre>
  
  <p>When user entered correct data user can proceed but still the error is there.I need to remove it!
  (I tried this far with lot of references so if i have miss anything please correct me.any new suggestions also appreciated)</p>