Handle panics from circuit construction
We handle them by reporting them to task that's waiting for the circuit, then relaying the panic.
Doing so allows the waiting task to distinguish panics (EK::Internal) from cases where the reactor dropped the task entirely (EK::ReactorShuttingDown). And doing that removes one case of EK::Canceled, which helps us on our goals towards #348 (closed).
Closes #347 (closed).