PRIORITY may be one of:
`debug' (messages useful for debugging upstart are logged, equivalent to --debug on kernel command-line);
`info' (messages about job goal and state changes, as well as event emissions are logged, equivalent to --verbose on the kernel command-line);
`message' (informational and debugging messages are suppressed, the default); `warn' (ordinary messages are suppressed whilst still logging warnings and errors);
`error' (only errors are logged, equivalent to --quiet on the kernel command-line) or
`fatal' (only fatal errors are logged).
Without arguments, this outputs the current log priority.
