Demo_Gen: Improved Error Generation

[ambiguousname]

While I was trying to set this up in the ICU4X repo, clearer errors would have been much more useful:

• I think it'd be good to allow generation even if certain errors (maybe just push warnings instead?) are present, so someone could quickly set up a demo and see how it works.
• Better warnings/error description would be good. Maybe showing the tree of dependencies, so people know where to look.

[ambiguousname]

I forgot that errors.set_context_method automatically handles some of this for us. Not exactly the stack I want, but we only set this at the start of a method. Which I would say is fairly useful.

What would help even more though is not including javascript in the error messages, because that's going to confuse a lot of people