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/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():