feature #27247 [Form] Deprecate searchAndRenderBlock returning empty string (ostrolucky)

This PR was squashed before being merged into the 4.2-dev branch (closes #27247).

Discussion
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[Form] Deprecate `searchAndRenderBlock` returning empty string

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #26531
| License       | MIT
| Doc PR        | -

I would like to remove this silent behavior, because it's confusing

Commits
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02f2f0ed39 [Form] Deprecate `searchAndRenderBlock` returning empty string

Author: fabpot

CHANGELOG.md

@@ -1,6 +1,11 @@
 CHANGELOG
 =========
 
+4.2.0
+-----
+
+ * deprecated calling `FormRenderer::searchAndRenderBlock` for fields which were already rendered
+
 4.1.0
 -----
 

FormRenderer.php

@@ -132,6 +132,9 @@ public function searchAndRenderBlock(FormView $view, $blockNameSuffix, array $va
         $renderOnlyOnce = 'row' === $blockNameSuffix || 'widget' === $blockNameSuffix;
 
         if ($renderOnlyOnce && $view->isRendered()) {
+            // This is not allowed, because it would result in rendering same IDs multiple times, which is not valid.
+            @trigger_error(sprintf('You are calling "form_%s" for field "%s" which has already been rendered before, trying to render fields which were already rendered is deprecated since Symfony 4.2 and will throw an exception in 5.0.', $blockNameSuffix, $view->vars['name']), E_USER_DEPRECATED);
+            // throw new BadMethodCallException(sprintf('Field "%s" has already been rendered. Save result of previous  render call to variable and output that instead.', $view->vars['name']));
             return '';
         }