i have been able to store the error in a file by using FileOutputStream
FileOutputStream fos = null;
File file = new File("errors.txt");
try
{
fos = new FileOutputStream(file);
}
catch(IOException ioe)
{
System.err.println("redirection not possible: "+ioe);
System.exit(-1);
}
PrintStream ps = new PrintStream(fos);
((ChannelExec)channel).setErrStream(ps);