add custom 404 page

searx/templates/courgette/404.html

+{% extends "courgette/base.html" %}
+{% block content %}
+<div class="center">
+    <h1>{{ _('Page not found') }}</h1>
+    <p>{{ _('Go to <a href="/">search page</a>.') }}</p>
+</div>
+{% endblock %}

searx/templates/default/404.html

+{% extends "default/base.html" %}
+{% block content %}
+<div class="center">
+    <h1>{{ _('Page not found') }}</h1>
+    <p>{{ _('Go to <a href="/">search page</a>.') }}</p>
+</div>
+{% endblock %}

searx/templates/oscar/404.html

+{% extends "oscar/base.html" %}
+{% block content %}
+<div class="text-center">
+    <h1>{{ _('Page not found') }}</h1>
+    <p>{{ _('Go to <a href="/">search page</a>.') }}</p>
+</div>
+{% endblock %}

searx/templates/pix-art/404.html

+{% extends "pix-art/base.html" %}
+{% block content %}
+<div class="center">
+    <h1>{{ _('Page not found') }}</h1>
+    <p>{{ _('Go to <a href="/">search page</a>.') }}</p>
+</div>
+{% endblock %}

searx/webapp.py

                     'default_theme': settings['ui']['default_theme']})
 
 
+@app.errorhandler(404)
+def page_not_found(e):
+    return render('404.html')
+
+
 def run():
     app.run(
         debug=settings['general']['debug'],

tests/robot/test_basic.robot

     Page Should Contain  about
     Page Should Contain  preferences
 
+404 page
+    Go To  http://localhost:11111/no-such-page
+    Page Should Contain  Page not found
+    Page Should Contain  Go to search page
+
 About page
     Click Element  link=about
     Page Should Contain  Why use Searx?