MEDIUM: mworker: exiting when couldn't find the master mworker_proc element

MEDIUM: mworker: exiting when couldn't find the master mworker_proc element

When a master process is reloading, the HAPROXY_PROCESSES variable is
deserialized. In older version of the master-worker (< 1.9), no master
element was existing in this variable.

This is not suppose to happen anymore, and could have provoked problems
in the master anyway.

This patch changes the behavior by exiting the master with an alert if
mp master element was found in this variable.