Soften the requirements for hooks execution

Currently, an unreachable node (or one that return undetermined failure)
in the hooks pre-phase will abort the curren operation. This is not
good, as a down node could prevent many operation on the cluster.

This patch changes a RPC-level failure (and not a hook execution
failure) into a warning. It also modifies the related test cases.

This fixes issue 11.

Reviewed-by: ultrotter