Adapted logging to the proposed loxberry plugin PHP standard.

[jo-math]

See https://wiki.loxberry.de/entwickler/entwicker_tipps_und_tricks/loxberry_logging_tips_and_tricks . Introduced improved error handling, as there were PHP systems warnings in certain conditions. If response code is not 4xx or 5xx, the data should not be interpreted, as the error contains error data.