Propagate error message to user on systemexit

[wbh1]

A previous PR added 'raise x from y" syntax, but consequently stopped raising
the useful error message content up the stack to the user.

This keeps the 'raise from' syntax, but just adds the error message back in.

Fixes: #2057

[ssbarnea]

My impression was that raise x from y was keeping that information. Is there something special about SystemExit that makes it fail to do so?

Can someone include a link about this? I am asking as I want to read more about the subject and avoid similar problem in the future.

[wbh1]

@ssbarnea it's because SystemExit doesn't print out a traceback [1], which is when __cause__ is actually used.

raise x from y sets the __cause__ variable on an object inheriting from BaseException. That __cause__ is used for chaining exceptions in the traceback printed to console [2], but since SystemExit never displays a stacktrace the __cause__ never ends up mattering.

I put this gist together to show the different behaviors.