Ability to override error messages

Lots of error messages are set by CiviCRM by dong stuff like:

$e =& CRM_Core_Error::singleton( );
$e->push( 9001, 0, null, 'We were unable to process your payment at this time. Please try again later.'); <-- This one is inside a payment processor.

for every message, there is an error code (9001 in this case). Assuming every error has a distinct code, there should be an easy way for people to override these error messages - perhaps a hook_civicrm_errorMessage() or something like that.

Currently, the only way (that I can find) of overriding these messages is to hack up the core code (well, use the Custom PHP directory).