New helper class created to serve it properly.
2. Added exception re-raise semantics.
Syntax: EXCEPTION;
If there was handled exception, re-initiate it, otherwise evaluate to no-op.
3. Implemented run-time exception messages.
Syntax: EXCEPTION <exception_name> [<value>];
If <value> is specified, evaluate it and use instead of RDB$EXCEPTION_MESSAGE.
4. Added new SQLCODE and GDSCODE system variables.
Available in procedures/triggers only.
If there wasn't any exception raised, return zero (success), otherwise return an error code.
5. Implemented ROWS_AFFECTED system variable.
Available in procedures/triggers only.
Count rows affected by the last INSERT/UPDATE/DELETE statement.
For any other statement, result is always zero.
