Fix unhandled rejection handling

[DABH]

Winston 3 supports catching/handling unhandled promise rejections. However, it looks like the implementation had some issues (which are probably my fault). Here is the story, for posterity:

We first noticed that we couldn't upgrade mocha past 8.1.3 (even to 8.2.0) without making our basic rejection handler test fail. I found that between these two versions (mochajs/mocha@v8.1.3...v8.2.0), some code involving listeners was added, and the effect seems to be that mocha adds an unhandled rejection listener to the process, which wasn't present in prior versions (they don't seem to add one for exceptions, don't ask). So we ensured we were removing the handler mocha added (TODO: it's probably best if we add it back after our test is done, though seems to be harmless to not have it).

However, we then saw that we couldn't make the test fail by messing around with the asserts. In other words, it seemed like the unhandled rejections weren't really getting handled by our handler. This led to some step-by-step debugging through the winston callchain to see where these rejections were going.

Eventually we realized that rejection objects (via the getAllInfo wrapper) were being treated like exceptions -- we used an ExceptionStream wrapper that filtered for, essentially, exceptions rather than rejections. We added a RejectionStream wrapper class and made necessary changes, and then everything was finally flowing smoothly.

In summary, the handler test for rejection handlers in winston basically wasn't checking anything (or at least not the intended thing) before, and we're not sure how broken rejection handling really was given this finding.

Oh, and we can now use the latest mocha version...

[DABH]

This doesn't get used anywhere atm but for completeness it felt like we should have this to parallel exceptions right above?

[DABH]

Scarily the tests passed locally even without a3e7ce8 ^^ 😬

[DABH]

This is the line in question. If possible, it's probably best to make a copy of mocha's listener, then remove all, then run the test, then remove all again at the end of the test (i.e., ensure the one we add during the test is removed) and finally re-add mocha's.