Add UI for handling the case when an enterprise has reached its maximum seat count

endpoints/index.py

@@ -112,7 +112,12 @@ def create_user():
   else:
     # New user case
     profile.debug('Creating user')
-    new_user = model.create_user(username, password, user_data['email'])
+    new_user = None
+
+    try:
+      new_user = model.create_user(username, password, user_data['email'])
+    except model.TooManyUsersException as ex:
+      abort(402, 'Seat limit has been reached for this license', issue='seat-limit')
 
     profile.debug('Creating email code for user')
     code = model.create_confirm_email_code(new_user)