[merge] different return code for internal errors

[Michael Hudson]

Martin Pool wrote:
> Bazaar has the concept of internal vs user errors.  The first type
> cause a traceback to be displayed and basically correspond to
> situations that we didn't expect to happen.  The latter are things
> that are (thought to be) not a bug in Bazaar but rather a problem
> in the environment or the user's input.

When I was writing tests for a fix recently, I was somewhat surprised
that this wasn't already the case.  So +1 from me :-)

Cheers,
mwh