SQL*XL does not block the system errors.
 
If an expected error is returned sometimes SQL*XL applies a workaround or a more meaningful advice message.  
If the error is truely unexpected it is displayed through the error handlers and it is offered to send the error in for analysis to us.
 
If you have a scenario that does not adhere to the above scenarios please let me know what the precise scenario is so I can investigate. I need the driver, database type and the scenario that led to the error.
 
The intention of the implementation in SQL*XL is to not bother the user when a workaround is known, to notify the user with a helpful message it is a known problem and to offer to send the technical details to us for analysis if there was an unknown error.