Handle yamllint_config issues with error messages

[bjhargrave]

If the specified yamllint_config is empty or malformed, we want to supply an appropriate error message.

Closes #60

[reidliu41]

seems it will show only failed msg if invalid, may no need to go next:

        lines = result.stdout.splitlines()
        logger.debug(f"====================xx=================> lines: {lines}")
        
        DEBUG 2024-10-24 09:38:13,551 instructlab.schema.taxonomy:286: ====================xx=================> lines: ['invalid config: not a dict']

[bjhargrave]

seems it will show only failed msg if invalid, may no need to go next:

I don't understand your comment. Can you be more specific? Thanks

[reidliu41]

sorry, not that clear. It should only show the invalid if failed, so may no need to check it in the next step, like pattern, the for-loop.

[bjhargrave]

It should only show the invalid if failed, so may no need to check it in the next step, like pattern, the for-loop.

In the loop we have established that the list exists and is non-empty. The output will either be empty (no messages because everything was fine, so the loop will not run), a list of messages (which will match the pattern, so the elifs will be ignored), or some other message such as invalid config or a traceback due to malformed config (in this case we are in "error" mode so no need for performance concerns and the elifs return so the loop is terminated). So I think the changes in this PR are correct.