Error handling is managed in source:trunk/classes/error.php

This class is always included in the Admin interface.
To raise an error, call the function UserError passing a text message (in english, it's not going to be translated by the Translator class) and an optional array of variables that can be useful for debugging.

If you want to call a function suppressing its possible errors, do not use the @ character in front of the function, as it will not work.
Use instead

set_error_handler("ErrorTrash");
$var = test_function();
set_error_handler("ErrorHandler");