This hasn't got much to do with ProWesS. It is sometimes useful to get the error string corresponding to an error code. REPORT will only work by printing to a channel, but in ProWesS you're not supposed to have a screen channel open, other than the implicit channels used by ProWesS itself, which are not accessible from SBasic. So you can't use REPORT. You could include a function in each of your programs, returning the error code strings, but you would have to write it in several languages if you wish to 'export' your program.
string$= ERRSTR$(error_code%)
incomplete$=ERRSTR$(-1)