-- для сообщения исключения с заранее известным CHARSET по умолчанию ОС клиента
CREATE DOMAIN CLIENT_MESSAGE AS VARCHAR(1023) CHARACTER SET WIN1251;
-- workaround for malformed string
CREATE DOMAIN EXCEPTION_MESSAGE AS BLOB CHARACTER SET NONE;

SET TERM ^;
CREATE OR ALTER PROCEDURE CONVERT_EXCEPTION_MESSAGE
  (
    "msg" CLIENT_MESSAGE
  )
  RETURNS
  (
    RET EXCEPTION_MESSAGE
  )
AS
BEGIN
  RET = "msg";
  SUSPEND;
END^
SET TERM ;^

-- Форма отстрела исключения
DECLARE VARIABLE msg VARCHAR(200) CHARACTER SET UTF8 DEFAULT 'Тестовое ';
...
EXCEPTION my_exception
  (SELECT RET FROM CONVERT_EXCEPTION_MESSAGE(:msg || 'сообщение'));

SELECT RDB$GET_CONTEXT('SYSTEM', 'ENGINE_VERSION') FROM RDB$DATABASE

CREATE OR ALTER EXCEPTION "my_exception" 'ID''s conversion exception';

SET TERM ^;
CREATE OR ALTER PROCEDURE oneRow
  RETURNS
  (
    tmstmp TIMESTAMP
  )
AS
  DECLARE VARIABLE msg VARCHAR(200) CHARACTER SET UTF8
    DEFAULT 'Тестовое ';
BEGIN
  EXCEPTION "my_exception" :msg || 'сообщение';
  tmstmp = CURRENT_TIMESTAMP;
  SUSPEND;
END^
SET TERM ;^

SELECT RDB$GET_CONTEXT('SYSTEM', 'ENGINE_VERSION') FROM oneRow;