remove unused function put_table and commented-out reference to it

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
fix conn.table_exists

The API call it was using does not exist.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
append.py: only continue if the file exists.

It will be much easier on the user this way.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>