Allow all rejection types.

In `RSVP.onerrorDefault` we only handled `instanceof Error`, but we really only care about ignoring `TransitionAborted` (because that is the only acceptable rejection).

This ensures that `String` errors are still handled by `RSVP.onerrorDefault`.
Errors that are not instances of Error so not necessarily have a stack property. This can cause a problem below when the error.stack is logged.