Previously all errors were 404 errors, even if the real error had
nothing to do with a file being non-existent.

Fixes #10283

Change-Id: I5b08b471a9064c347510cfcf8557373704eef7c0
Reviewed-on: https://go-review.googlesource.com/9200Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>