the specifics of how php error handling is set up is something you should consult with your host on.
In many cases the host will have custom error handling files to handle 404, or 500 errors to control what is displayed to the user when an error occurs.
They will also set up error messages to display when the server is down.
the errors will be stored in a log file, and the server can be configured to alert you through email if an error occurs.
none of these things are handled by Power Store however, they are handled by the PHP server that power store is installed on. consulting with your host will be the best way to ensure that errors are handled the way you wish.
The power store forms do have validation in place to prevent incorrect data from being entered and will display an error message, this is displayed even before it gets sent to the MySQL server.
the inserts and updates where designed in such a way that there should not be any consequences on failure.
For example, if the order insert is successful, but inserting the order details fails, the order will be left in the database, but it will not be displayed as an order and will not effect the available inventory.
If a user is updating the database at the same tie an email is being sent, the first person to submit will have precidence.