Properly check whether a table exists in _getSchema
Properly check whether a table exists in _getSchema

The return code check was merely signaling a server-side error, and was not
actually a reliable indication for the table not existing. This patch instead
checks properly by doing a select on the schema_columnfamilies table.

gwicke/restbase-mod-table-cassandra,eevans/restbase-mod-table-cassandra