Make shell only print error/warning log once.

Without this change, the shell would always print the error log twice
for successful non-insert queries (once in __execute_query() and once
in __fetch()).

Change-Id: I0ab038230df897559b30feaea34778ea72988bc3
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2815
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins