sync: fix print error when handling server error

When converting this logic from print() to the output buffer, this
error codepath should have dropped the use of the file= redirect.

Bug: https://crbug.com/gerrit/14482
Change-Id: Ib484924a2031ba3295c1c1a5b9a2d816b9912279
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305142
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>