Complain on invalid log levels. Previously only first log level was required to be correct, while error_log directive in fact accepts list of levels (e.g. one may specify "error_log ... debug_core debug_http;"). This resulted in (avoidable) wierd behaviour on missing semicolon after error_log directive, e.g. error_log /path/to/log info index index.php; silently skipped index directive and it's arguments (trying to interpret them as log levels without checking to be correct).