Summarization error when switching between documents in fluid preview app

[vladsud]

I believe the reason for the error - me switching documents (actually - creating a new doc while old one was summarizing).
I think it would be nice not to issue error (i.e. category="error") as this is not really an error, but rather normal flow of events. Or better - keep summarizing :)

[arinwt]

Easy fix is to handle the promise rejection of timer cancel (or change PromiseTimers to resolve and handle that flow).

Keeping summarizer client running I would mark as related to #931. Though this part could be done independently, if we break the simple concept of keeping summarizer client alive only while main client is alive, then we should at least be thinking about consequences.

[arinwt]

Mostly addressed in #1729. Specifically, the "UnexpectedSummarizeError" mentioned above. Leaving this issue open, because there is one other place it can happen, and probably should just change how timers work to never reject. This might mean changing the API of timers as well, or at least the async PromiseTimers.

[arinwt]

This should be fixed by #2368 and #770 which changed PromiseTimer to always resolve.