After 85e3c7b087 `processResponse` method
sets `err` field or sets status in `statusRes` field of `contextCall`.
In previous implementation `ObjectWriter.Close` method returned
`ctxCall.err` on `false` return of `processResponse` method. This could
cause NPE-panic if status failure resolving was disabled.
Make `ObjectWriter.Close` to return internal `err` field only if it is
set, otherwise return status response.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
ea21cdf731