Don't re-raise StepFailure exceptions in cbuildbot main script.
Don't re-raise StepFailure exceptions in cbuildbot main script.

StepFailure exceptions are already recorded in the report and in the
stage that failed, so there is no need to print them again.

BUG=chromium-os:32596
TEST=Run cbuildbot and verify tracebacks are still printed out, but
     not quite as many times.

Change-Id: Id1c5c537de1151d3f26ff28f50306b3cbd7a45de
Reviewed-on: https://gerrit.chromium.org/gerrit/27460
Commit-Ready: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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