garbage_collection: only ignore 'missing chunk' errors

garbage_collection: only ignore 'missing chunk' errors

with the fix for #2909 (improving handling missing chunks), we
changed from bailing to warning during a garbage collection when
updating the atime of a chunk.

but, updating the atime can not only fail when the chunk is missing,
but also on other occasions, e.g. no permissions or more importantly,
no space left on the device. in that case, the atime of a valid and used
chunk cannot be updated, and the second sweep of the gc will remove that chunk.
[0] is a real world example of that happening.

instead, only warn on really missin chunks, and bail on all other
errors.

0: https://forum.proxmox.com/threads/pbs-server-full-two-days-later-almost-empty.83274/

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>