Add error_bad_chunk to the JSON loader
Add the `error_bad_chunk` parameter to the JSON loader.

Setting `error_bad_chunk=False` allows to skip an unparsable chunk of JSON data without raising an error.

Additional note:

In case of an unparsable JSON chunk, the JSON loader no longer tries to load the full JSON (which could take a lot of time in streaming mode) to get the JSON fields that the user may have forgotten to pass. Ex : for squad-like data, the user has to pass `field="data"` to tell the loader to get the list of examples from this field.

TODO: update docs

cc @lvwerra 