It looks like the shell only displays the error message, but not the stack trace, which is silently swallowed. I created the following issue against the apache project, so you can vote for it there.
Actually I just got word from Chris who is looking into it that it's a Felix Karaf issue now after the move, but he'll take care of it nevertheless.
What is your timeframe for a resolution? We could have a quick fix for fuse esb, but in this case I think we should let it take the normal course and get it fixed at apache first.
I do have a work around (hack) for now.
entire start() method ...
} catch (Throwable t)
Please let know when its fixed.