To use the functions described in this section, you will need to load
the (lepton log) module.
Emit a log message with the specified level. The message is interpreted as a format string, with format-args as its parameters.
The level should be one of the symbols error,
critical, warning, message, info or
debug.
Note: if level is set to error, main program will terminate.
Generally, if the level is not debug, then the message
should be localised.