Uemis downloader: don't ignore return values from fs operations

In most cases the existing code might have done the right thing, but some
of the hard to reproduce errors might actually stem from the fact that we
have intermittend fs errors. Maybe this addresses things?

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Null terminate buffer at the correct spot

fread tells us how many bytes it read.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Mark ignored error return as intentional

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>