Handle errors raised by plugins more gracefully in mod_wsgi.
    
    This started as an effort to display a more useful error message in the
    Apache error log if retrieving the schema failed. I broadened the scope
    a little to include limiting the output in the Apache error log
    so errors are easier to find.
    
    This adds a new configuration option, startup_traceback. Outside of
    lite-server.py it is False by default so does not display the traceback
    that lead to the StandardError being raised. This makes the mod_wsgi
    error much easier to follow.