Make the output of final exception subscriber developer friendly

It is interesting that \Drupal\Component\Render\PlainTextOutput::renderFromHtml does something similar, but not the same.

This is all some legacy code inherited from the days when we tried to render exceptions in a drupal themed way.

1. +++ b/core/lib/Drupal/Core/EventSubscriber/FinalExceptionSubscriber.php
   @@ -119,12 +119,14 @@ public function onException(GetResponseForExceptionEvent $event) {
   -        $message = new FormattableMarkup('%type: @message in %function (line %line of %file). <pre class="backtrace">@backtrace</pre>', $error);
   +        $message = new FormattableMarkup("%type: @message in %function (line %line of %file).\n\n@backtrace", $error);
   

   I'm wondering about removing the class and pre tags. Does this impact devel? I.e do we need a change record?

2. +++ b/core/lib/Drupal/Core/EventSubscriber/FinalExceptionSubscriber.php
   @@ -119,12 +119,14 @@ public function onException(GetResponseForExceptionEvent $event) {
   +      $content .= "\n\n" . strip_tags(htmlspecialchars_decode($message, ENT_QUOTES));
   

   I think we should use \Drupal\Component\Render\PlainTextOutput::renderFromHtml() here - so we standardise on how we convert HTML to plain text.

3. Also where are you seeing the HTML. If a look at an exception in the browser it looks like this
   
   
   So are we fixing this on the right level - if you're seeing this in Drush or Console I think they should problem fix this.

Re-rolled the patch #27.

Since #2924860: “Uncaught exception” page contains HTML but has content-type text/plain landed this is now making the the output less not more developer friendly. I think we we should consider doing something like:

    $content = $this->t('The website encountered an unexpected error. Please try again later.');
    if ($event->getRequest()->getRequestFormat() == 'html') {
      $content .= $message ? '<br/><br/>' . $message : '';
      $content_type = 'text/html';
    }
    else {
      $content .= $message ? "\n\n" . strip_tags(PlainTextOutput::renderFromHtml($message)) : '';
      $content_type = 'text/plain';
    }
    $response = new Response($content, 500, ['Content-Type' => $content_type]);

That way html output gets all the advantage of html and properly escaped errors and plain output looks good but has all HTML tags removed for the error message as we cannot be sure about the context it might be displayed it.

Comments addressed in #35 & added an interdiff.

Missed to include use Drupal\Component\Render\PlainTextOutput; in patch. Updated patch added with an interdiff.

is this issue make the taxonomy term view page down related
?

The website encountered an unexpected error. Please try again later.LogicException: The controller must return a response (null given). Did you forget to add a return statement somewhere in your controller? in Symfony\Component\HttpKernel\HttpKernel->handleRaw() (line 169 of /webroot/vendor/symfony/http-kernel/HttpKernel.php).

2019/07/07 05:05:46 [error] 30704#30704: *1582 FastCGI sent in stderr: "PHP message: Uncaught PHP Exception LogicException: "The controller must return a response (null given). Did you forget to add a return statement somewhere in your controller?" at /webroot/vendor/symfony/http-kernel/HttpKernel.php line 169" while reading response header from upstream, client: 111.222.122.122, server: abc.doamin, request: "GET /taxonomy/term/10318 HTTP/2.0", upstream: "fastcgi://127.0.0.1:9999", host: "abc.domain"

with drupal core 8.7.4 and distribution varbase

https://www.drupal.org/project/varbase/issues/3066163

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Here I have tried to add reroll of patch #38 for Drupal 9.1.x.