Fix errors due to non-unpicklable Exception when "enqueue=True"

[Delgan]

Fix #329.
Fix #504.

There are instances where a custom Exception might contain attributes that cannot be serialized or is incorrectly implemented (see Cannot unpickle Exception subclass). If this occurs with enqueue=True, an error will be raised by the background thread when it calls queue.get(). Due to the failure of de-serialization, the record is None, resulting in an inability to identify the origin of the error.

To address this concern, we can take a proactive step by attempting to unpickle the Exception beforehand. In the event of a failure, we replace its value with None to make sure it can be unpickled. This strategy mirrors what we already do for the serialization part. Actually, this approach was already implemented in the past. However, it was temporarily removed due to concerns raised about its safety (#563). Subsequent assessments have demonstrated its complete safety, allowing us to reintroduce it. Furthermore, we have expanded its scope to encompass all types of exceptions, not solely limited to UnpicklingError. This broader implementation serves to mitigate the aforementioned issues effectively.