LUNodeEvacuate: Locking fixes

When evacuating a node, only an assertion without informative text was
used to check if the necessary node locks had been acquired. This was on
top of evaluating the list of nodes without having a node group lock, so
this was changed as well.

Also update some exception messages to include “retry the operation”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>