Add 503 error handler

app/templates/errors/503.html.j2

@@ -0,0 +1,19 @@
+{% extends "base.html.j2" %}
+
+{% block page_content %}
+<div class="container">
+  <h1 id="title">{{ title }}</h1>
+  <p class="light">{{ request.path }}</p>
+  <p>Alternatively, you can visit the <a href="{{ url_for('main.index') }}">Main Page</a> or read <a class="modal-trigger" href="#more-information-modal">more information</a> about this type of error.</p>
+</div>
+
+<div class="modal" id="more-information-modal">
+  <div class="modal-content">
+    <h2>About the "{{ title }}" error</h2>
+    <p>The server cannot handle the request (because it is overloaded or down for maintenance). Generally, this is a temporary state.</p>
+  </div>
+  <div class="modal-footer">
+    <a href="#!" class="btn-flat modal-close waves-effect waves-green">Close</a>
+  </div>
+</div>
+{% endblock page_content %}