minor #15077 [HttpFoundation] Use convention to allow throwing from _…

…_toString() (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[HttpFoundation] Use convention to allow throwing from __toString()

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

Corollary to #15076, works without it.

Commits
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8982c32 [HttpFoundation] Use convention to allow throwing from __toString()

src/Symfony/Component/HttpFoundation/Request.php

@@ -480,7 +480,7 @@ public function __toString()
         try {
             $content = $this->getContent();
         } catch (\LogicException $e) {
-            trigger_error($e->getMessage(), E_USER_ERROR);
+            return trigger_error($e, E_USER_ERROR);
         }
 
         return