When openPartialFile is invoked, more errors than "existed error" are
returned. If only existing error is checked, the allocated field of
partialFile may be nil.
Signed-off-by: Star.LI <star@trapdoortech.com>
We were ignoring quite a few error cases, and had one case where we weren't
actually updating state where we wanted to. Unfortunately, if the linter doesn't
pass, nobody has any reason to actually check lint failures in CI.
There are three remaining XXXs marked in the code for lint.
