Catching errors

Is it possible to "catch" errors in pl/pgsql? For
example, instead of the function exiting with (from an
INSERT attempt):

ERROR:  duplicate key violates unique constraint
"user_name"
CONTEXT:  PL/pgSQL function "add_user" line 12 at SQL
statement

The function could catch the error, then generate it's
own custom error message:

raise exception ''user name already exists'';

Thanks,
CSN



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