Exit gracefully with informative message when no arguments are passed

Summary:
Alternatively, we could set required=True in the click argument. This would give:
```
➜  Desktop dagster schedule start
Usage: dagster schedule start [OPTIONS] SCHEDULE_NAME...
Try "dagster schedule start --help" for help.

Error: Missing argument "SCHEDULE_NAME...".
```
See: https://click.palletsprojects.com/en/7.x/arguments/#variadic-arguments

Test Plan: Manual

Reviewers: sashank, prha

Reviewed By: sashank

Differential Revision: https://dagster.phacility.com/D2351

python_modules/dagster/dagster/cli/schedule.py

@@ -241,11 +241,17 @@ def extract_schedule_name(schedule_name):
 
 
 @click.command(name='start', help="Start an existing schedule")
-@click.argument('schedule_name', nargs=-1)
+@click.argument('schedule_name', nargs=-1)  # , required=True)
 @click.option('--start-all', help="start all schedules", is_flag=True, default=False)
 @repository_target_argument
 def schedule_start_command(schedule_name, start_all, **kwargs):
     schedule_name = extract_schedule_name(schedule_name)
+    if schedule_name is None and start_all is False:
+        print(
+            'Noop: dagster schedule start was called without any arguments specifying which '
+            'schedules to start. Pass a schedule name or the --start-all flag to start schedules.'
+        )
+        return
     return execute_start_command(schedule_name, start_all, kwargs, click.echo)
 
 

python_modules/dagster/dagster_tests/cli_tests/test_cli_commands.py

@@ -748,6 +748,17 @@ def test_schedules_start_and_stop(_patch_scheduler_instance):
     assert 'Stopped schedule foo_schedule\n' == result.output
 
 
+def test_schedules_start_empty(_patch_scheduler_instance):
+    runner = CliRunner()
+
+    result = runner.invoke(
+        schedule_start_command, ['-y', file_relative_path(__file__, 'repository_file.yaml')],
+    )
+
+    assert result.exit_code == 0
+    assert 'Noop: dagster schedule start was called without any arguments' in result.output
+
+
 def test_schedules_start_all(_patch_scheduler_instance):
     runner = CliRunner()