cassandra-count should error out on C* 2.0, or fix the dependency on size_estimates table

[weideng1]

It appears that if you try to use cassandra-count against a C* 2.0 table, because 2.0 doesn't have system.size_estimates table yet, it will error out with some obscure error messages. We should either remove the dependency on this system table information, or catch this error and prevent cassandra-count from running against a C* cluster lower than version 2.1.