Check the help topic: "Defining the error message using SRW.MESSAGE" in reports:
In PL/SQL, you can define an error message and raise it in Report Builder message region with a PL/SQL packaged procedure, SRW.MESSAGE. (Note: You can also define and raise errors within the user exit program. For more information, see Raising errors from the user exit.
SRW.MESSAGE has the following syntax:
SRW.MESSAGE (msg_number NUMBER, msg_text CHAR);
Hope this helps..
- G.Sharma