Warn on tables with missing cells

Shouldn't tidy warn about tables with missing cells?
```
   <table border="1">
      <tr>
        <td>A</td>
        <td>B</td>
      </tr>
      <tr>
        <td>1</td>
      </tr>
    </table>
```
The boss will say "Great presentation, Williams" until he sees our
mangled table... if indeed the browser gets that far trying to render it!
