Improve error message when conversation isn't available in Module\Item\Display

- Retrieve the parent post no matter what. Previously it was depending on the user's settings, which gave unpredictable behavior if the parent post wasn't retrieved for whatever reason.
- List possible reasons the conversation isn't showing up based on the condition in Model\Post::selectViewForUser
- Allow HTML in exception messages

view/templates/exception.tpl

@@ -1,7 +1,7 @@
 <div id="exception" class="generic-page-wrapper">
     <img class="hare" src="images/friendica-404_svg_flexy-o-hare.png"/>
     <h1>{{$title}}</h1>
-    <p>{{$message}}</p>
+    <p>{{$message nofilter}}</p>
 {{if $thrown}}
 	<pre>{{$thrown}}
 {{$stack_trace}}