Fail builds on Paygen failures.
Fail builds on Paygen failures.

If the PaygenStage fails, fail the build.

Also, add a clearer exception to describe crostools import errors.

BUG=chromium:345721
TEST=Unittests + tryjob (lumpy-release)

Change-Id: I41e5571bdc146387af22930da8fb04653408b2c0
Reviewed-on: https://chromium-review.googlesource.com/189484
Tested-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Reviewed-by: Peter Mayo <3d31fdaf44dc55bb1b4f50b19df9bc879177d37a@chromium.org>
Commit-Queue: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>

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