1. Remove an invalid error check.
2. Make sure to shadow the outer error type from within the goroutine instead or
reading the outer type.
This may have been causing test issues (caught in TestMinerAllInfo with the race
detector).
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>
