[hooks/syncschema] do not crash on double removal of a constraint
Some complicated migrations may entail a double constraint removal.
It is not yet known whether this is a genuine cw bug or the result
of a complicated migration story, but we should nevertheless
not crash.
Rather, emit a CRITICAL warning and proceed.
Closes #3595309.
<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>