urisourcebin: Avoid deadlock on shutdown

urisourcebin: Avoid deadlock on shutdown

The reason why the STATE lock was taken was to avoid issues where we would be
adding (and activating) elements at the same time as urisourcebin would be
brought down to READY. That would cause those new elements to potentially return
ERRORS because of not-negotiated/flushing-pads

But that creates a really bad deadlock (state lock is taken to deactivate the
streaming thread which .. is currently grabbing the state lock).

Instead, we can just ignore the warning/error messages that might occur when
shutting down.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/4075

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8187>