maintner: suppress spurious error message

Currently maintnerd prints

    2017/06/17 15:00:16 Unknown error type <nil>: <nil>

if it successfully downloads a file. It shouldn't log if there was no
error.

Change-Id: I01c4f2bde616d709d5c081219f9c123d6ea6a678
Reviewed-on: https://go-review.googlesource.com/46007
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed