unavailability looks silly

Currently when a file is unavailable the text "File not found" is printed into the Availability column and this text comes before the most freshly published files in the listing.

We should set availability of unavailable files to 0 seconds. This way they will be shown last in the ordering. Then we should change the textual representation of 0 seconds to "n/a". We should however not change the function that renders time intervals into strings, as that function will be used for ETA counters also, and 0 seconds with and ETA counter certainly does not mean unavailability.