Merge branch '5.1' into 5.2

* 5.1:
  [Uid] fix checking for valid UUIDs
  [HttpKernel] Configure the ErrorHandler even when it is overriden
  Allow relative path to composer cache

Author: nicolas-grekas

EventListener/DebugHandlersListener.php

@@ -30,6 +30,7 @@
  */
 class DebugHandlersListener implements EventSubscriberInterface
 {
+    private $earlyHandler;
     private $exceptionHandler;
     private $logger;
     private $deprecationLogger;
@@ -51,6 +52,10 @@ class DebugHandlersListener implements EventSubscriberInterface
      */
     public function __construct(callable $exceptionHandler = null, LoggerInterface $logger = null, $levels = \E_ALL, ?int $throwAt = \E_ALL, bool $scream = true, $fileLinkFormat = null, bool $scope = true, LoggerInterface $deprecationLogger = null)
     {
+        $handler = set_exception_handler('var_dump');
+        $this->earlyHandler = \is_array($handler) ? $handler[0] : null;
+        restore_exception_handler();
+
         $this->exceptionHandler = $exceptionHandler;
         $this->logger = $logger;
         $this->levels = null === $levels ? \E_ALL : $levels;
@@ -78,6 +83,10 @@ public function configure(object $event = null)
         $handler = \is_array($handler) ? $handler[0] : null;
         restore_exception_handler();
 
+        if (!$handler instanceof ErrorHandler) {
+            $handler = $this->earlyHandler;
+        }
+
         if ($handler instanceof ErrorHandler) {
             if ($this->logger || $this->deprecationLogger) {
                 $this->setDefaultLoggers($handler);