[RP] error message from C-t !

[Shawn Betts]

Hi everyone,

I've changed the code around a bit to give an error message if a
command failed to execute. ratpoison prints the message 'Command not
found'. So now if you type something like:

C-t ! emacsss

then ratpoison will yell at you, saying:

Command not found

Ideally it should say 'Command "emacsss" not found' but that would
have required a bunch more hacking :). For now, this is what you get.

Let me know if you guys have any problems on platforms other
than GNU Libc (that's all I tested on).

Shawn