Fix errors & warnings for compose blocks

[dfremont]

Description

While debugging I noticed that the warning for modular scenarios whose compose block is stuck doesn't work, since I forgot a couple of imports. It turns out we have a test for stuck behaviors but not for stuck scenarios. So I've fixed the bug and added a test.

The error message for compose blocks that don't invoke any subscenarios (or use wait) also didn't work, so I've fixed that and added a test.

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

[lola831]

Looks good. I tested this locally, and the new test passes. The CI failure appears to be from an unrelated flaky test, not from the changes in this PR, so I'm going ahead and merging.