Try to prevent instance memory changes N+1 failures

There are multiple bugs with the code checking for N+1 failures in the
instance memory changes which needs significant changes, in the
meantime we can at least:

- change the warning message into an error (--force will skip checks)
- only make checks when we increase the memory

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