fix: ensure email retrieval and order processing in EventHandler

This commit addresses two main issues in EventHandler.php. Firstly, the fetching of the email from the GuestOrder session is moved at a different location in the flow where it makes more sense and doesn't get overridden prematurely.

Secondly, an exhaustive condition handling has been added to address cases where there might be a payment error or async payment error. The method fetches the current order via the URL hash which could be an OrderInProcess or an already existing Order.

The commit also ensures that the customer UUID is set correctly if the session exists. An update is made to the database to accommodate this change.

Minor changes have also been made to spacing and data-type declaration in GuestOrderUser.php to help with readability and consistency.

Related: #11 (closed)