Telling shell command errors from command exit statuses.

[Kenneth Almquist]

The shell has (presumably) already forked off a child process by the
time it discovers that a command does not exist.  Therefore, the only
way it *can* indicate that the command does not exist is to return
a nonzero exit status.  Perhaps a special exit status should be reserved
for "command does not exist".
				Kenneth Almquist