Improve certain error messages in portage_utilities.EBuild.
Improve certain error messages in portage_utilities.EBuild.

Changed selected calls to cros_build_lib.Die() so that the error
message is more specific about the failure, and includes information
that is needed to diagnose the problem.

BUG=None
TEST=manually provoke the errors, and read the messages.

Change-Id: Ib49fb08980dda4ed09f51b4868f487a0acba21d0
Reviewed-on: https://gerrit.chromium.org/gerrit/12571
Commit-Ready: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>
Tested-by: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Thieu Le <8cae94ed37a002427b56783feab9ab75ab7aa0b8@chromium.org>

zhang0137/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,coreos/chromite