Custom Error function - Extend Exceptions or add to Common? Confused

[eluser]the real rlee[/eluser]
Hi guys, I would like to add a new error function which i can call anywhere in my CI app much like show_error(). I've read some posts about extending the Exceptions library, but it seems show_error() is a wrapper for the Exceptions show_error() function so I'm assuming the only way to create a custom error function which i can call just like show_error() is to ad it directly the Common.php file (?)