Add special errors for bad chat/ift types

[milocress]

Adds the InvalidExampleTypeError and InvalidMessageTypeError for when example is not a Mapping and message is not an Iterable, respectively.

[dakinggg]

Can you manual test this and make sure the error is raised as expected?

[milocress]

Can you manual test this and make sure the error is raised as expected?

hf-nopeft-noseqpar-8b-noloadpath-ayi0lW for the extraneous keys:

[rank0]: UnknownExampleTypeError: Found keys {'response', 'prompt', 'random_extra_key'}
[rank0]: in dataset. Unknown example type. For prompt and response finetuning, the valid
[rank0]: prompt keys are {'prompt'} and the valid response keys are {'response',
[rank0]: 'completion'}. For chat finetuning, the allowed keys are {'messages'}

hf-nopeft-noseqpar-8b-noloadpath-4yxvUW for bad example type:

raises MisconfiguredHfDatasetException

hf-nopeft-noseqpar-8b-noloadpath-fIpZH0 for bad messages type:

raises MisconfiguredHfDatasetException

Because HF catches the misconfiguration before foundry, I added some unit tests to make sure the correct error is raised in the UC case.