Let's say some package does
(make-thread (lambda () (message "Started thread.") (error "Fit happens.") (message "Finished thread.")))
M-x toggle-debug-on-error, the error will be silently ignored. Even on
(thread-join (make-thread ...))
the error will not be handled in any manner.
(thread-last-error) can give access to the error object, but it is subject to race conditions.
Is it possible to run the debugger instead, when there is an error in threads?