add HTML templates for 404/500 errors

cms/templates/404.html

+<%inherit file="base.html" />
+<%! from django.core.urlresolvers import reverse %>
+<%block name="title">Page Not Found</%block>
+
+<%block name="content">
+
+<div class="wrapper-content wrapper">
+  <section class="content">
+
+    <h1>Page not found</h1>
+    <p>The page that you were looking for was not found. Go back to the <a href="/">homepage</a> or let us know about any pages that may have been moved at <a href="mailto:technical@edx.org">technical@edx.org</a>.</p>
+  </section>
+</div>
+
+</%block>
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cms/templates/500.html

+<%inherit file="base.html" />
+<%! from django.core.urlresolvers import reverse %>
+<%block name="title">Server Error</%block>
+
+<%block name="content">
+
+<div class="wrapper-content wrapper">
+  <section class="content">
+      <h1>Currently the <em>edX</em> servers are down</h1>
+      <p>Our staff is currently working to get the site back up as soon as possible. Please email us at <a href="mailto:technical@edx.org">technical@edx.org</a> to report any problems or downtime.</p>
+  </section>
+</div>
+
+</%block>
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