[Followup] Merge system.fast_404 config object into system.performance

Patch merges the configs and adds an upgrade path test.

Actually using update_variables_to_config for these variable is interesting as in D7 they are declared in settings.php and D8 they can be in config... but must read original conversion issue to see if this was discussed :) #1778478: Convert fast_404 to CMI - doesn't appear so... but this issue should be for a follow up. Relatedly, I'm not sure why we still have the comments in defaults.settings.php (imho) this settings should just be added to the system performance ui.

+++ b/core/lib/Drupal/Core/ExceptionController.phpundefined
@@ -138,7 +138,7 @@ public function on404Html(FlattenException $exception, Request $request) {
     // Check for and return a fast 404 page if configured.
-    $config = config('system.fast_404');
+    $config = config('system.performance')->get('fast_404');
 
     $exclude_paths = $config->get('exclude_paths');

Doh! $config is no longer a config object... this ain't gonna work :)

Also forgot to update default.settings.php

Why do I always forget this :(

$ git apply --index 1934716.fast_404.4.patch 
error: patch failed: core/modules/system/tests/upgrade/drupal-7.system.database.php:119
error: core/modules/system/tests/upgrade/drupal-7.system.database.php: patch does not apply

Sorry, this is not applying for me.

Needed reroll due to #1821420: Convert mail variables to cmi making unrelated whitespace changes to drupal-7.system.database.php

When green this should be back to rtbc

Committed and pushed to 8.x. Thanks!

Patch in #8 had a typo in the default.settings.php file that affects all generated settings.php files; if the fast_404 option is enabled, the missing end quote in 'system.performance' leads to a PHP syntax error.

#12: 1934716-fast-404-default-settings-typo.patch queued for re-testing.

Looks good.

Nice catch... and oops :)

Committed c750885 and pushed to 8.x. Thanks!