use .flatten instead of .map_err(...)?

[tdyas]

In #22688, the Result<_, JoinError> returned by (the renamed) native_spawn_blocking was handled with a map_err. As it turns out, the error closures passed into the (now deleted) spawn_blocking were to handle the JoinError. The code should convert any JoinError's with a single map_err and then flatten so that any Result returned by the task is unwrapped.