Make the output of final exception subscriber developer friendly

The exception response has been turned into text/plain last year for the purpose of security (SA-CORE-2016-004). For this reason HTML entities are now shown unprocessed. I propose we strip out HTML tags and decode special chars to make the error messages more readable.

Before:

The website encountered an unexpected error. Please try again later.</br></br><em class="placeholder">LogicException</em>: The controller must return a response (null given). Did you forget to add a return statement somewhere in your controller? in <em class="placeholder">Symfony\Component\HttpKernel\HttpKernel-&gt;handleRaw()</em> (line <em class="placeholder">171</em> of <em class="placeholder">vendor/symfony/http-kernel/HttpKernel.php</em>). <pre class="backtrace">Symfony\Component\HttpKernel\HttpKernel-&gt;handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session-&gt;handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle-&gt;handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache-&gt;pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache-&gt;handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware-&gt;handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware-&gt;handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel-&gt;handle(Object, 1, 1) (Line: 656)
Drupal\Core\DrupalKernel-&gt;handle(Object) (Line: 19)
</pre>

After:

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 171 of vendor/symfony/http-kernel/HttpKernel.php).

Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 656)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)