Better example of logging

stderr gets the two worst log levels,
dtrace can see them all:

$ sudo dtrace -Zs logging.d -c ./usdt_logger.py
error message
critical message
python57961:dtrace-logger:logging:debug 10 debug message
python57961:dtrace-logger:logging:info 20 info message
python57961:dtrace-logger:logging:warning 30 warning message
python57961:dtrace-logger:logging:error 40 error message
python57961:dtrace-logger:logging:critical 50 critical message
python57961:dtrace-logger:logging:notset 1 notset message