Clarify lock check assert message if another session is locked (#13473)

Picking this to the 7.2 branch since the other part of the ticket was already included in 7.2.

Change-Id: I1120ad5acd553e22db95e3635fffbd453fd26310
(cherry picked from commit 134c3bb96bfeaf1eab488e685f3b5dce3093e0ef)