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).

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