BUG-8511: add more explicit messages

This adds more defensive handling of connections and locking,
even if it should not strictly be necessary, as we are using
atomic operations and run on the actor thread. This makes the
transitions work even in fact of actor context leakage.

Change-Id: I26df0f208d63b861a0f3d3dc3c0f1959bbc79e90
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 90a377ec4fe7c1aa4df6d4fde74bfc8189e95b08)