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* Add example failing task

This task includes an invalid import. This will cause an exception and
the task will not be loaded. But this just results in a DEBUG level log
message, so in normal usage you'll see no error, and will be told the
task doesn't exist.

Here's an example command line to run the task:

    python -m lm_eval --model hf --model_args pretrained=rinna/japanese-gpt-1b --tasks fail

This task is based on a Japanese Winograd task, but that's not
important, and was just used due to familiarity.

* Do not ignore errors when loading tasks

* Change how task errors are logged

This makes the proposed changes from PR discussion.

1. Exceptions not related to missing modules/imports are logged as
   warnings.

2. module/import related exceptions are still logged at debug level, but
   if any of them happen there is a warning about it with instructions
   on how to show logs.

* Remove intentionally failing task

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Co-authored-by: Paul O'Leary McCann <polm@dampfkraft.com>