BUG/MEDIUM: mworker: more verbose error upon loading failure

BUG/MEDIUM: mworker: more verbose error upon loading failure

When a worker crashes during its configuration parsing and without
emitting any messages, the master will emit the message "Failed to load
worker!". However that doesn't give us neither the PID of the worker,
nor the status code.

This patch fixes the problem by emitting a more verbose error.

Must be backported as far as 3.1.

(cherry picked from commit 67cb6aab9006f19cd6478040fb8adc2f7c6153d6)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 65a7be02035a3bf9ad6176601da222d17e6676a6)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>