Errors in "message" eval should be handled more helpfully

[bgribble]

If you are entering a literal Python value or expression in a Message and make a typo, you get a error traceback and theobject is thrown into a weird state. It can be worked through, but it would be nice if this was a little cleaner: maybe just put up a badge with an error indicator and a HUD message indicating a syntax error, and no stack trace?