stream/cache: handle failure of seeking underlying stream

This could for example happen when serving an incomplete file from http,
and the demuxer tries reading data from the end of the file when opening
it (e.g. with avi). Seeking past EOF fails with http, so the file could
never be opened, and the cache would get stuck trying to seek to the
position.

We can't really make the cache report seek failure directly (it would
suck for various reasons), so just make the cache report EOF if seeking
fails.

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