Poisoned thread leads to confusing panic in Pool::current

[gerwin3]

In this code:

sharded-slab/src/shard.rs

Lines 292 to 294 in 40579b9

	let tid = Tid::<C>::current();
	test_println!("current: {:?}", tid);
	let idx = tid.as_usize();

Tid::current().as_usize() may return usize::MAX if the thread was poisoned. This makes it hard to debug since the error seems to indicate the user spawned to many threads, which is not the case.