log chained exceptions

[eltonlaw]

fixes #1350

[eltonlaw]

Here's how it looks right now (using Flask)

[davidism]

There's still some issues here. It looks like we don't want to use chain=True in exception, since that sticks the whole traceback at the top and bottom of the page, instead of just the exception. Instead, we want to make plaintext understand the chained exceptions.

Started digging into this and basically we need to create groups of frames and let them handle the formatting. Python actually has a similar idea, but it's in 3.5+ only. I'm working on it.

[davidism]

Rebased your original commmit, then made changes as described in the previous comment. Did a bunch of compat cleanup in tbtools too.

A lot of the processing moved from Traceback to the new Group object. Made the style for the chain info message look like the "Traceback (most recent call last)` header.

[davidism]

Turns out that frame.f_lineno is not accurate (at least in chained exceptions), and traceback.tb_lineno should be used instead. I could not find an explanation for this difference.