Add error handling

This adds an /error/XXX endpoint that triggers the corresponding XXX
error code and its handling.

Related: https://github.com/alphagov/notifications-aws/pull/331

app/__init__.py

@@ -488,6 +488,10 @@ def register_errorhandlers(application):  # noqa (C901 too complex)
     def handle_gone(error):
         return _error_response(410)
 
+    @application.errorhandler(413)
+    def handle_payload_too_large(error):
+        return _error_response(413)
+
     @application.errorhandler(404)
     def handle_not_found(error):
         return _error_response(404)

app/main/views/index.py

@@ -1,4 +1,4 @@
-from flask import redirect, render_template, request, url_for
+from flask import redirect, render_template, request, url_for, abort
 from flask_login import current_user, login_required
 from notifications_utils.international_billing_rates import (
     INTERNATIONAL_BILLING_RATES,
@@ -18,6 +18,11 @@ def index():
     return render_template('views/signedout.html')
 
 
+@main.route('/error/<int:status_code>')
+def error(status_code):
+    abort(status_code)
+
+
 @main.route("/verify-mobile")
 @login_required
 def verify_mobile():

app/templates/error/413.html

@@ -0,0 +1,21 @@
+{% extends "withoutnav_template.html" %}
+{% block per_page_title %}File too big{% endblock %}
+{% block maincolumn_content %}
+    <div class="grid-row">
+      <div class="column-two-thirds">
+        <h1 class="heading-large">
+          The file you uploaded was too big
+        </h1>
+        <div class="grid-row">
+          <div class="column-two-thirds">
+            <p>
+              Files must be smaller than 5 MB.
+            </p>
+            <p>
+              <a href="javascript: history.go(-1)">Go back and try again.</a>
+            </p>
+          </div>
+        </div>
+      </div>
+    </div>
+{% endblock %}