custom error messages/numbers

Folks,

I am developing an app for PostgreSQL, entirely with stored functions (somewhat resembling the object-oriented approach. In fact it is not an app, just an API). All my functions return an integer value - 0 if the function was successful, another value otherwise.

Until now, all parameter checking was done manually in the stored function (if param is null then return -8; end if; etc.) This makes things quite lengthy.

I know about check constraints. However, upon error they throw an error message and abort the current transaction, So my question is: is there any possible way of throwing my own error messages upon a check constraint violation?

Thank you very much

Zoltan Bartko