Fix CSS classes in error pages and show custom error message on 403

aleksis/core/templates/403.html

@@ -5,10 +5,16 @@
 {% block content %}
   <div class="container">
     <div class="card red">
-      <div class="card white-text">
-        <i class="material-icons small">error_outline</i>
-        <span class="card-title">{% trans "Error" %} (403): {% blocktrans %}You are not allowed to access the requested page or
-          object.{% endblocktrans %}</span>
+      <div class="card-content white-text">
+        <i class="material-icons small left">error_outline</i>
+        <span class="card-title">
+        {% if exception %}
+          {{ exception }}
+        {% else %}
+          {% trans "Error" %} (403): {% blocktrans %}You are not allowed to access the requested page or
+          object.{% endblocktrans %}
+        {% endif %}
+        </span>
         <p>
           {% blocktrans %}
             If you think this is an error in AlekSIS, please contact your site

aleksis/core/templates/500.html

@@ -6,7 +6,7 @@
   <div class="container">
     <div class="card red">
       <div class="card-content white-text">
-        <div class="material-icons small">error_outline</div>
+        <div class="material-icons small left">error_outline</div>
         <span class="card-title">{% trans "Error" %} (500): {% blocktrans %}An unexpected error has
           occured.{% endblocktrans %}</span>
         <p>

aleksis/core/templates/503.html

@@ -6,7 +6,7 @@
   <div class="container">
     <div class="card red">
       <div class="card-content white-text">
-        <div class="material-icons small">error_outline</div>
+        <div class="material-icons small left">error_outline</div>
         <span class="card-title">{% blocktrans %}The maintenance mode is currently enabled. Please try again
           later.{% endblocktrans %}</span>
         <p>