Support error response

[tavurth]

https://developers.databox.com/responses-errors/

The above are not currently supported, even though they are present in your documentation.

These errors are silently dropped by the push function here:

databox-python/databox/__init__.py

Lines 96 to 106 in d28806f

	self.last_push_content = self._push_json({
	'data': [self.process_kpi(
	key=key,
	value=value,
	date=date,
	unit=unit,
	attributes=attributes
	)]
	})
	return self.last_push_content['id']