from_http better exception handling

[cumason123]

Expected Behavior

from_http(data="", headers={})

This should throw CloudEventMissingFields("Invalid data field") or similar descriptive error

Actual Behavior

First, from_http attempts to read this event as a structured event.

Because there is nothing inside data, it fails to unmarshal'. Therefore it runs json.loads("") because it expects data to be a json string with cloud event extensions.

Finally you get a json decoder error which is difficult to debug

[cumason123]

In general, the repository needs better exception handling to help users debug invalid cloudevents easily.