MINOR: mworker: replace ha_alert by ha_warning when exiting successfuly

MINOR: mworker: replace ha_alert by ha_warning when exiting successfuly

Since commit bb86986 ("MINOR: init: report the haproxy version and
executable path once on errors") the master-worker displays its version
and path upon a successful exits of a current worker. Which is kind of
confusing upon a clean exits.

This is due to the fact that that commit displays this upon a ha_alert()
which was used during the exit of the process.

Replace the ha_alert() by an ha_warning() if the process exit correctly
and was supposed to.

It still displays the message upon a SIGINT since the workers are
catching the signal.