Suppress output unless non-zero exit code

You're going to have to buffer the output somewhere no matter what, since you need to wait for the exit code to know what to do. Something like this is probably easiest:

$ output=`my_long_noisy_script.sh 2>&1` || echo $output

The moreutils package contains a program chronic for this purpose. You just call it like

chronic my_program args ...

Very handy in cron jobs.