AppleScript Runner exit status passed back to shell script
The -e flag prints errors to stderr and is the default. So you just need to read stderr.
This answer might help you if you aren't familiar with that:
bash variable capture stderr and stdout separately or get exit value
EDIT: Added sample code.
error=`osascript -e 'tell app "Finder" to adtivate' 2>&1`echo $errorThe above on my system captures the error messages.