This fixes some issues with aqt.taskman.TaskManager:

• TaskManager.run_in_background() can raise an exception if there was an exception in a (possibly unrelated) background task that was just completed. When this happens, the new background task is not run and the exception bubbles up to the code that called run_in_background().
  • This can lead to confusing issues. For example, we encountered an issue where the central exception handler of the AnkiHub add-on was raising an exception after trying to start a background task (for uploading logs). As a fix, we wrapped the run_in_background() calls in the exception handler with run_on_main(). We encountered similar issues with this before.
• When there are multiple closures in TaskManager._closures, and one of the closures raises an exception, the other closures are not called at all

Proposed changes

• In TaskManager._on_closures_pending(), catch exceptions and re-raise them using QTimer.singleShot with a delay of 0 instead of allowing them to bubble up immediately.
  • Effects:
    • All closures are run
    • The caller of run_in_background() is not interrupted by exceptions from closures and the new background task is scheduled successfully
    • All exceptions from closures are raised

Risks

• Maybe some code relies on the current taskman exception-handling behavior?
  • If making this change is too risky, it could be adjusted. However, it would be good to prevent exceptions from bubbling up to the caller of run_in_background(), or at least document that this can happen.