Added back button for errors

Added a single href link to the error.html page in datasette/templates. allows users to go back without having to use the browser's back button. temporary fix.

datasette/templates/error.html

@@ -1,11 +1,10 @@
 {% extends "base.html" %}
 
 {% block title %}{% if title %}{{ title }}{% else %}Error {{ status }}{% endif %}{% endblock %}
-
 {% block content %}
 
 <h1>{% if title %}{{ title }}{% else %}Error {{ status }}{% endif %}</h1>
 
 <div style="padding: 1em; margin: 1em 0; border: 3px solid red;">{{ error }}</div>
-
+<a href="javascript:window.history.back();">Return to Previous</a>
 {% endblock %}