ipc: improve exception handling

ipc: improve exception handling

When an exception triggers a teardown of the worker process
(ipc_worker_stop), we need to combine subsequent exceptions and
show the original one, first.  In other words, we must not lose
the original exception if new exceptions are thrown during
teardown.

So rely on `wantarray' to grab caller contexts to allow
returning exceptions as a list rather than throwing them
immediately.