Make the Sys_Error() message more informative

Make the Sys_Error() message more informative

Re: 24550510e6ff331c1749707df7fad608f618bdd3 as discussed in chat, there
was a good reason to change this word: it indicates DP deliberately
stopped execution, whereas "Error" is generic (most problems that can
happen on a computer are some form of error).  Also "Engine Error"
makes it sound like there's a bug in DP, which isn't necessarily the
case: there are several scenarios where user error can trigger this.
This message was already different from Quake's "Quake Error".

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>