[fetch] capture stderr to reduce extraneous noise.
[fetch] capture stderr to reduce extraneous noise.

This should hide scary-looking but innocuous stderr noise under normal
operation.

R=dnj@chromium.org, phajdan.jr@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2841423002

luci/recipes-py,luci/recipes-py