[BUGFIX beta] Don’t assert uncaught RSVP rejections

Fixes #9901.

We already loudly log a stack trace, and the assert
ends up breaking everything else on the run loop
queue and given that it doesn’t even preserve a
meaningful stack trace related to the source of
error, it doesn’t add a lot of value (and causes
real problems a la #9901).
Merge pull request #9904 from machty/no-rsvp-error-assert

[BUGFIX beta] Don’t assert uncaught RSVP rejections