Death messages from mobs are grammatically incorrect

Relates to MC-119905

When killed by an unnamed mob, the death message displays as
"<username> was slain by <Mob>"

One would expect this to look like one of these instead:
"<username> was slain by a <Mob>"
"<username> was slain by a <mob>"

Named mobs, as well as other players, currently produce grammatically correct messages.

This could be fixed by adding a separate death message for being killed by an unnamed mob, whereas the current death message would be kept for named mobs as well as players.