std: Don't fail the task when a Future is dropped

It's a benign failure that no one needs to know about.

Closes #14892
Clever use of TLS. I didn't realize you could use it that way to find out if the task exited due to failure or not.

Seems to me you can skip the atomics though, and just use a separate channel. Have Bomb send `task::failing()` across a channel and read it on the other side.