Handle AlreadyExistsError on user creation with non-unique email

templates/error.html

@@ -0,0 +1,11 @@
+{% extends "error_base.html" %}
+
+{% block content %}
+
+<main class="usa-section usa-content">
+
+<h1>{{ message }}</h1>
+
+</main>
+
+{% endblock %}