var StdErr: Text;
The StdErr variable is connected to the standard error file handle. To report errors, you should prefer WriteLn (StdErr, 'everything wrong') over WriteLn ('everything wrong').
StdErr is a GNU Pascal extension.
program StdErrDemo; var Denominator: Integer; begin ReadLn (Denominator); if Denominator = 0 then WriteLn (StdErr, ParamStr (0), ': division by zero') else WriteLn ('1 / ', Denominator, ' = ', 1 / Denominator) end.