Add a template for error pages

The template is just a page but add a `<base href="/">` tag to make sure
that resources for the page will always be loaded from the root.

This fixes missing CSS and JS when encountering an error in a
subdirectory of the site (ie.  http://example.org/blog/doesnt_exist)

To use it, add `template: error` to the metadata of the error page
diff --git a/templates/base.html b/templates/base.html
index 941d77a..437d3ac 100644
--- a/templates/base.html
+++ b/templates/base.html
@@ -7,6 +7,7 @@
   <meta name="author" content="{{ AUTHOR }}">
   <meta name="description" content="{% block description %}Posts by {{ AUTHOR }}{% endblock %}">
 
+  {% block base %}{% endblock %}
   {% assets filters="pyscss,cssmin", output="css/style.%(version)s.min.css", "css/style.scss", "css/pygments.css" %}
   <link rel="stylesheet" type="text/css" href="{{ SITEURL }}/{{ ASSET_URL }}">
   {% endassets %}
diff --git a/templates/error.html b/templates/error.html
new file mode 100644
index 0000000..98c1ada
--- /dev/null
+++ b/templates/error.html
@@ -0,0 +1,5 @@
+{% extends "page.html" %}
+
+{% block base %}
+  <base href="/">
+{% endblock %}