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.