add Exception type for MultiError.catch

[WindSoilder]

which added for feature #408 . And change some relative unit test

[njsmith]

Hey, thanks for jumping in to work on this!

Implementing the ideas in #611 is going to be a bit complicated. We're going to have to make several compatibility-breaking changes:

• nurseries start raising MultiErrors, even if there's only one exception
• MultiError.catch starts taking the exception type (as in this PR)
• MultiError.catch starts calling the handler once with a combined exception, instead of a bunch of times on individual exceptions

Also it'd be nice to coordinate a bit with the asyncio folks before committing to something here, since we're hoping to converge with them on this part.

Given all that, I think we probably want to move a little slowly here, and try to batch up all these changes together, instead of doing multiple rounds of compatibility-breaking changes. So, I'm afraid I'm rejecting this PR in its current form.

#607 is somewhat similar in terms of refactoring some of trio's core APIs, and there's a pretty concrete plan for what to do (especially in this comment: #607 (comment), once you dig through all my verbiage). Or if you want to keep working on the MultiError stuff, I have a half-written prototype locally that I'll try to at least push up somewhere in the next few days, and we could work together on that?

[WindSoilder]

Hi, Sorry for the late reply..
It's very great for me to keep working on the MultiError stuff.

And, of cause I can also try to help on #607 when I have time :)