requests.Response objects evaluate as falsy when the status_code
attribute is in the 400-500 range. Therefore we are assured that
prior to this change, APIError would show `is_server_error() == False`
when generated with a 500-level response and `is_client_error() == False`
when generated with a 400-level response. This is not desirable.

Added some seemingly dry (not DRY) unit tests to ensure nothing silly
slips back in here.
Signed-off-by: alex-dr <alex@datarobot.com>