Carbon intensity fetchers should fail gracefully when external API's return statuscode 400

[Snailed]

When external API's return non-200 status codes, a JSON deserialization error is thrown.
This can be seen in issue #64.

Expected behavior is a relevant network error message for the user to act accordingly upon.

The problem can be seen in the following lines (can also be seen in the other fetchers)

carbontracker/carbontracker/emissions/intensity/fetchers/energidataservice.py

Lines 43 to 48 in 2d2b34d

	def _emission_prognosis(self, time_dur):
	from_str, to_str = self._interval(time_dur=time_dur)
	url = "https://api.energidataservice.dk/dataset/CO2Emis?start={" + from_str + "&end={" + to_str + "}&limit=4"
	response = requests.get(url)
	if not response.ok:
	raise exceptions.CarbonIntensityFetcherError(response.json())

The code throws an exception when computing response.json(), and thus does not raise a CarbonIntensityFetcherError as intended.