error codes used by subroutine task_error:

normal or info messages:       1xx, 2xx
warnings:                      3xx, 4xx
minor errors:                  5xx, 6xx
errors:                        7xx, 8xx

The fai-monitor-gui has 4 icons for the error levels.

The variable $STOP_ON_ERROR (default 700) will define that return
codes that are greater than this value will stop the installation.
