[views] implement a custom json error view
If the client asked for ``(e)jsonexport`` view, answer
him with a json response rather than an error formatted
in a big HTML blob.
add a custom _requested_vid predicate. In cw 3.21, it will
be deprecated in favor of ``match_form_params(vid=('jsonexport', 'ejsonexport'))``
<table class="attributeForm" style="width:100%;"
tal:attributes="id tab_id | nothing;
class tab_class | nothing;">
<tr tal:iter="widget lines">
<th class="labelCol" tal:content="structure python:widget.render_label(entity)">attrname</th>
<td tal:define="error python:widget.render_error(entity)" style="width:100%;"
tal:attributes="class python:error and 'error' or nothing">
<div tal:replace="structure error">error message if any</div>
<div tal:replace="structure python:widget.edit_render(entity)" >widget (input, textarea, etc.)</div>
<div tal:replace="structure python:widget.render_help(entity)">format help if any</div>
</td>
</tr>
</table>