PHD: fail active serial console waits if the serial websocket connection is lost

The serial console wait logic (see below) has no affordance to stop waiting and return an error if the serial console connection is lost.

https://github.com/oxidecomputer/propolis/blob/5a6a3323a8a586fda836b2945f3bf07881b5b04a/phd-tests/framework/src/serial/vt100.rs#L122-L131

While wait-for-serial operations have a timeout, this can be long (5 minutes in the wait-to-boot case), so it's a bummer to have to wait it out if the VM is just totally gone (e.g. because the test did something that made the server panic).

The serial console websocket listener (below) can detect this case and send a message on its output_tx telling the VT processor (which actually handles the waits) that the connection is gone, allowing the listener to send a failure message/close its end of the channel/whatever behavior we decide is most appropriate.

https://github.com/oxidecomputer/propolis/blob/5a6a3323a8a586fda836b2945f3bf07881b5b04a/phd-tests/framework/src/serial/mod.rs#L29-L41

Start with the wait logic in phd-tests/framework/src/serial/vt100.rs around lines 122-131, then read the serial websocket listener in phd-tests/framework/src/serial/mod.rs around lines 29-41. Trace how output_tx reaches the VT processor and determine how a lost connection should report failure. Done means active serial waits fail promptly instead of waiting for their timeout.