Reword the error message for engine errors

It's been reported that the existing error messages could be confusing
to users. In particular, the notion of a computer program dying (error
message when the engine process is killed) could be confusing. Reword to
avoid this.
Rephrase the player death message

For whatever reason, PGN has a notion of player death, to be used if the
moons align and a player is unable to complete a game because he died.
Probably no gnome-chess user will ever see this message ever -- he would
have to load a specially-prepared PGN -- but let's make it less
confusing anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=726454