Plugin crashes are not always properly detected

[jvanstraten]

This is probably two issues, but I haven't investigated it fully yet. Ultimately what can happen is that DQCsim hangs when a plugin fatally crashes, specifically when an upstream plugin desyncs with a downstream plugin. So, two problems:

• The crashing plugin should propagate its error message upstream.
• Even if a plugin crashes so hard that it doesn't send this message, DQCsim should not wait indefinitely. It should periodically timeout its message fetching logic to check that all plugins are still alive, and propagate an error upstream if they aren't.