IMPALA-931: Indicate in impala-shell --help that verbose output is enabled by default

Change-Id: I6c29d542c4379d46fcdce62905c89b319628dd61
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2842
Reviewed-by: Abdullah Yousufi <abdullah.yousufi@cloudera.com>
Tested-by: jenkins
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
Fix more decimal overflow issues.

Change-Id: Ie88a6dc1a584ce795831b3880abb63c967b8c471
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2835
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins