Mobly exits with status code 0 after TestAbortAll

[puradox]

If a test executed by test_runner raises signals.TestAbortAll before any failures or errors are received, the test will exit with status code 0 instead of what I expected to be 1. This is due to not executing ok = False

https://github.com/google/mobly/blob/991ecb2d9f7f859f8d86ab771d991473c30f80bc/mobly/test_runner.py#L71-L87

To make this more confusing, if a test failure or error is experienced before TestAbortAll is raised, the test will exit with status code 1.

I believe both of these behaviors oppose most users' expectations of the implication of aborting a test. I would expect the test to always return status code 1 whenever an abort occurs. WDYT about adding ok = False to the except signals.TestAbortAll statement?

At the very least, some doc comments above each signal type would be valuable for setting expectations.