You would still get the warnings if you redeirected STDERR to STDOUT but if you trust the called script to exit non-zero when it fails then you no longer need to examine STDERR. You can throw it away with 2>&/dev/null and just test the exit value from the script.

Fixing the warnings may also help ;-)

The bad interpreter error would also return a non-zero value to the calling script and get caught in the $? test.

Cheers,
R.