upas/marshal: handle nonexistent save folder correctly (thanks sirjofri)

upas/marshal: handle nonexistent save folder correctly (thanks sirjofri)

When the save folder did not exist, and we could not create
it, we would handle up to one Biobuf worth of message, and
then fail, due to a failed tee. The sequence of events leading
up to this was:

	openfolder() -> error
	tee(0, fd, -1) -> wait for read
	write(0, data) ->
		write(fd, data) -> ok
		write(-1, data) -> error, tee terminates
	write(0, attachment) -> error

This change prevents us from writing to a closed fd, and
therefore from erroring out when sending.

We also warn the user.