cros_chrome_make: add error message when ebuild invocation fails.
cros_chrome_make: add error message when ebuild invocation fails.

BUG=chromium-os:36711
TEST=ran manually

Change-Id: Ic55692bed5c41eabd79978843ace445e65553c94
Reviewed-on: https://gerrit.chromium.org/gerrit/38832
Reviewed-by: asharif <0bffdc695497b480334d6a40467f7e340b20eba1@google.com>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

coreos/dev-util,marineam/coreos-dev-util,marineam/coreos-dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,mgireesh05/dev-util