Improve error when run with no command given

[jacalata]

e.g "tabcmd --server --site --..." would throw an exception about not having an attribute.
Now it will print out the usage information.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tabcmd
__main__.py	10	10	10	0%
tabcmd.py	14	14	14	0%
version.py	6	3	3	50%
tabcmd/commands
commands.py	10	10	10	0%
constants.py	77	15	15	81%
server.py	130	20	20	85%
tabcmd/commands/auth
session.py	360	42	42	88%
tabcmd/commands/datasources_and_workbooks
datasources_and_workbooks_command.py	127	16	16	87%
datasources_workbooks_views_url_parser.py	140	10	10	93%
delete_command.py	60	16	16	73%
export_command.py	114	25	25	78%
get_url_command.py	127	47	47	63%
publish_command.py	87	24	24	72%
runschedule_command.py	21	7	7	67%
tabcmd/commands/extracts
create_extracts_command.py	42	8	8	81%
decrypt_extracts_command.py	27	2	2	93%
delete_extracts_command.py	37	6	6	84%
encrypt_extracts_command.py	27	2	2	93%
extracts.py	20	2	2	90%
reencrypt_extracts_command.py	27	2	2	93%
refresh_extracts_command.py	49	13	13	73%
tabcmd/commands/group
create_group_command.py	29	5	5	83%
delete_group_command.py	27	2	2	93%
tabcmd/commands/project
create_project_command.py	46	8	8	83%
delete_project_command.py	35	4	4	89%
publish_samples_command.py	30	4	4	87%
tabcmd/commands/site
create_site_command.py	34	5	5	85%
delete_site_command.py	28	2	2	93%
edit_site_command.py	38	2	2	95%
list_command.py	44	10	10	77%
list_sites_command.py	29	2	2	93%
tabcmd/commands/user
create_site_users.py	57	10	10	82%
create_users_command.py	60	10	10	83%
delete_site_users_command.py	43	5	5	88%
user_data.py	221	31	31	86%
tabcmd/execution
_version.py	2	2	2	0%
global_options.py	146	23	23	84%
localize.py	69	11	11	84%
logger_config.py	42	6	6	86%
tabcmd_controller.py	42	8	8	81%
TOTAL	2712	444	84%

[anyoung-tableau]

Does this line need updating too? hasattr(namespace