Make multiplexer's "command not found" exit 127.

Make multiplexer's "command not found" exit 127.

Somebody (carsonh on freenode) objected that they were running something
like "toybox test 1 == 2" and couldn't distinguish the test failing from
toybox not having the test command. Why they were running toybox out of
the multiplexer instead of having a test symlink in the $PATH, I couldn't
tell you. (Is "I know toybox is on this system, but don't know how it was
configured" a problem people actually have?)

*shrug* They care about it more than I do, so now you can't distinguish between
the multiplexer not being found and the command the multiplexer tried to
run not being found. Progress!