Documentation for start_launch says it returns UUID or None but it could return NOT_FOUND in some cases

[vokiput]

Describe the bug
start_launch could return NOT_FOUND constant instead of UUID or None in some cases.

Steps to Reproduce
Steps to reproduce the behavior:

self.client = RPClient(endpoint='rp.endpoint.com', project='PROJECT_XXX', api_key='VALID_API_KEY_FOR_USER_WHO_DOES_NOT_HAVE_ACCESS_TO_PROJECT_XXX')
launch = self.client.start_launch(name='LAUNCH_NAME', start_time=timestamp())

Expected behavior
launch is None

Actual behavior
launch == NOT_FOUND

Package versions
5.5.10

Additional context
Nothing