sfeed_update: fail early if creating a temporary directory or status file fails

If creating a temporary directory for the feed files failed then $sfeedtmpdir
would be empty and it would try to:

	mkdir -p "/feed"
	touch "/feed/ok"

After failing it would also still try to process all the feeds.

Now just fail early.

mktemp or touch themselve will print the actual error to stderr.

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