cubicweb #2897836 Session may raise attribute _txs instead of SessionClosedError [validation pending]

prioritynormal
typebug
done in3.17.1
load0.500
load left0.000
closed by#dcd8aa47adce [session] raise proper exception with get_tx is called on a closed session
patch[session] raise proper exception with get_tx is called on a closed session [applied]