Errors in expressions

Hi,

This should fix issue #6. It silently fails if the user doesn't have permission to fix it, so only a user with Manage portal will see an error message, the non working field is just skipped otherwise. The error is quite hard to spot, adding some highlighting would be useful, but the error css classes in Plone don't work on the tags being used, and I didn't want to add in some product level css. There is also a scope issue created if you try to change the css_class on error.

Cheers
Michael