[Smart Select] error message / item info

This is a (multiple allowed):

• bug
• enhancement
• feature-discussion (RFC)

Motivation:
To be consitent to the other input elements it would be great to have an error message / item info for smart select as well (-> enhancement). An error message is e.g. pretty useful if there is a dependency between multiple inputs (choosen value in smart select A does does not work if a certain value is choosen in smart select B).

Currently, framework7 (v2) does not not recognize the "item-after" div in case it is not a direct child of item-inner (-> bug?)

Expected Behavior / proposal

DOM structure like:

...
 <div class="item-content">
     <div class="item-inner">
         <div class="item-title">title</div>
         <div class="item-select-wrapper">
            <div class="item-after">selected value</div>
            <div class="item-input-error-message">errormsg</div>
         </div>
      </div>
   </div>

> smart select recognizes the "item-after" div within the wrapper correctly
> some css definition for item-inner-with-custom-error, items-select-wrapper
> update documentation (example)

Start by inspecting the Smart Select implementation and the existing handling of item-after and input error messages. Compare its rendered structure with the proposed wrapper, then verify the selected value and error message are recognized, the related CSS is covered, and the documentation example is updated.