Add signal to inform joint that body has exited tree

[madmiraal]

Currently there is no way to inform a Joint that a PhysicsBody attached to it has been removed. This PR connects the tree_exiting signal from a PhysicsBody attached to a configured Joint to provide this information and update the Joint.

Fixes #44510

Note: This also addresses:

1. The error message(s) raised in Bullet physics when trying to reconfigure a Joint that has had a PhysicsBody freed without first disconnecting it from the Joint; as described here.
2. The warning message raised in Bullet physics when a scene was closed with a PhysicsBody attached to a Joint above it in the tree; as identified here.

This PR includes raising the warning message in Bullet physics to an error; since this should now never occur.

[pouleyKetchoupp]

Everything seems to work fine now! It should be good to go after the last little coding style fix.

[akien-mga]

Thanks!