Improve error messages for misnested elements

[aantron]

From ocsigen/tyxml#322:

There is a popular way to style checkbox based on this HTML code:

  <label class="switch" for="checkbox">
    <input type="checkbox" id="checkbox" />
    <div class="slider"></div>
  </label>

What I did not realise is that this HTML, although popular, does not respect the specification (div should be span here).

TyXML does detect that and correctly rejects the program:

let%html checkbox =
  "<label class='switch' for='checkbox'>"
    "<input type='checkbox' id='checkbox' />"
    "<div class='slider'></div>"
  "</label>"

However, the error message is just wrong: Error: Unmatched start tag 'label', pointing at the beginning of the label element. It took me quite a long time to understand what was happening: it would be nice to have it pointing to the div element instead and state that it can't be within a label element.

I'm using TyXML 4.6.0 with tyxml-ppx.

cc @Mbodin