All kernels hang in UI if supervisor process is terminated

[jmcphers]

System details:

Positron and OS details:

Any version with supervisor enabled.

Interpreter details:

Any.

Describe the issue:

If the supervisor process ends for any reason, Positron doesn't respond gracefully. All the sessions stop responding without any notifications or user feedback.

It isn't likely that the process will end on its own, but it could exit abruptly in rare circumstances:

• Something triggers a close of all terminals (since the supervisor runs in a terminal)
• An OOM killer or other system level reaper forces the supervisor to terminate
• The supervisor itself panics and crashes due to an unexpected runtime condition

Steps to reproduce the issue:

Ensure the supervisor is enabled, then start a Python session. Get the process ID of the supervisor using Python's os library to get the parent process ID of the Python session:

>>> os.getppid()
84461

Then, kill that process (e.g. kill 84461 in a terminal).

Nothing appears to happen in the UI, but you can no longer execute code, LSPs are dead, you don't get diagnostics, etc.

Expected or desired behavior:

At the very least:

• A notification should be displayed to the user indicating that a crash has occurred, along with any output emitted by the supervisor that might be relevant (e.g. a backtrace)
• All affected sessions should enter the Exited state

More ambitiously, we could do better at crash recovery:

• The supervisor itself could run sessions in such a way that it could reattach after a crash
• We could attempt to restart all the sessions that were running before the crash

Implementation-wise, we might do something like check the process ID to see if it's still running if we see our websocket disconnect or an API call fails with a connection refused error unexpectedly, or we could have some sort of heartbeat for the supervisor itself.