Improve logging of unparseable S2 message

[lfse-slafleur]

Line 68 in router.py: https://github.com/flexiblepower/s2-analyzer/blob/main/backend/s2_analyzer_backend/router.py#L68

Should be writing s2_json_msg instead of s2_msg (it will always be None). Clean up with some extra newlines. Break up S2ValidationError instead of just dumping it immediately to string.