We finally got it working, but we found that the error messages were horrendous – even for someone who has been using free and open-source tools for a while now. Printing out errors from compiler or interpreter directly along with the stack trace is almost always the worst thing to do for a tool/utility (as opposed to an API or library). The stack trace is definitely useful, for developers trying to build upon or improve your tool. Have a debug or development mode where developers can get all the information they need.
If you care about your users, especially new users, make sure you spend sufficient time on showing human-readable messages. If possible list the possible causes for every error along with tips for troubleshooting.