[PATCH 2 of 2 RFC] ui: add support for fully printing chained exception tracebacks

[Bryan O'Sullivan]

On Sun, Jan 06, 2013 at 11:40:55PM -0500, Matt Harbison wrote:
> 
> Is there a better way to do this?  I thought about either replacing SubrepoAbort
> with a general error.ChainedAbort or slipping such a superclass in beneath
> SubrepoAbort, but it doesn't seem worth it since subrepos are the only place I
> know of attempting chained exceptions.

That seems reasonable enough for now.

> The print syntax looks funny, but I read something somewhere that
> print(...,file=...) is only supported in 2.6 and later.

Maybe use ui.write_err instead?