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/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 %}