Fix JSON error handling

[ange-daumal]

Fix to prevent KeyError on 'timeseries'.

If requested symbol is incorrect/null, Yahoo Finance API returns this response :

{'finance': {'result': None, 'error': {'code': 'Not Found', 'description': 'HTTP 404 Not Found'}}} 

Hence raising a KeyError exception instead of the YFinanceException one, with an explicit error message.

You can re-create the case this PR fixes by trying an incorrect ISIN code, e.g. yf.Ticker("FR0000000000").info