BUG/MINOR: hlua: don't call ha_alert() in hlua_event_subscribe()

BUG/MINOR: hlua: don't call ha_alert() in hlua_event_subscribe()

hlua_event_subscribe() is meant to be called from a protected lua env
during init and/or runtime. As such, only hlua_event_sub() makes uses
of it: when an error happens hlua_event_sub() will already raise a Lua
exception. Thus it's not relevant to use ha_alert() there as it could
generate log pollution (error is relevant from Lua script point of view,
not from haproxy one).

This could be backported in 2.8.