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>

maintner/netsource.go

@@ -74,6 +74,9 @@ func (ns *netMutSource) GetMutations(ctx context.Context) <-chan MutationStreamE
 // isNoInternetError reports whether the provided error is because there's no
 // network connectivity.
 func isNoInternetError(err error) bool {
+	if err == nil {
+		return false
+	}
 	switch err := err.(type) {
 	case *url.Error:
 		return isNoInternetError(err.Err)