show engine errors when no result is returned at all

searx/templates/oscar/messages/no_results.html

 {% if unresponsive_engines %}
 <div class="alert alert-danger fade in" role="alert">
     <p><strong class="lead">{{ icon('remove-sign') }} {{ _('Error!') }}</strong> {{ _('Engines cannot retrieve results.') }}</p>
+    <p>
+        {% for engine_name, error_type in unresponsive_engines %}
+        {{ engine_name }} ({{ error_type }}){% if not loop.last %}, {% endif %}
+        {% endfor %}
+    </p>
     <p><small>{{ _('Please, try again later or find another searx instance.') }}</small></p>
 </div>
 {% else %}