Improve error reporting for small clusters

When doing a two-node allocation on a cluster/group in which only one
node is online, or a one-node allocation without any online nodes, we
didn't show a valid error mesage. The patch changes tryAlloc to "fail
hard" in this case, to make the failure explicit.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>