Add status code cache context and use it.

Closes #3516173

core/lib/Drupal/Core/Cache/Context/ExceptionStatusCodeCacheContext.php

+<?php
+
+namespace Drupal\Core\Cache\Context;
+
+use Drupal\Core\Cache\CacheableMetadata;
+
+/**
+ * Defines the ExceptionStatusCodeCacheContext service.
+ *
+ * Cache context ID: 'exception_status_code'.
+ */
+class ExceptionStatusCodeCacheContext extends RequestStackCacheContextBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getLabel(): \Stringable {
+    return t('Exception status code');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContext(): string {
+    $exception = $this->requestStack->getCurrentRequest()->attributes->get('exception');
+    if ($exception) {
+      return (string) $exception->getStatusCode();
+    }
+    // If there's no exception status code, usually a 200, return '0' because we
+    // don't know what might be set by response subscribers.
+    return '0';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheableMetadata(): CacheableMetadata {
+    return new CacheableMetadata();
+  }
+
+}