backend: Don't throw error when EOS'ing an MPEG file

Bastien Nocera requested to merge wip/hadess/less-mpeg-errors into master

When seeking an MPEG file past the end, mpeg2dec would throw an error about "No valid frames decoded before end of stream". Catch that error and emit an EOS message instead, so that we don't block MPEG playback every second.

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