UU: DB: Exception differentiation

["Ilya A. Volynets-Evenbakh" <ilya@total-knowledge.com>]

Currently PostgreSQL doesn't support differentiating
between user-initiated exceptions, except by message.

However, it isn't the best way to differentiate exceptions
in C++. I'd like to come up with a mechanism for differentiating
UU exceptions in some uniform way.

Ideally, we'd have a common source of mapping code to
meaning, shared between stored procedures and C++
(or a common source, which would be preprocessed
to generate C++ header and postgresql script).

This way we could have easily maintainable error messages,
uniform status codes, and easy way to do i18n.

Any ideas on how to implement it in the best way
possible?

   Ilya.