For example, I inserted an exception in the CleanupStage, and obtained the following log:

@@@BUILD_STEP@BuildStart@@@
************************************************************
** Start Stage BuildStart - Thu, 15 Sep 2016 15:39:37 -0700 (PDT)
** 
** The first stage to run.
** 
**   This stage writes a few basic metadata values that are known at the start of
**   build, and inserts the build into the database, if appropriate.
************************************************************

@@@STEP_LINK@Builder documentation@http://www.chromium.org/chromium-os/build/builder-overview#TOC-Pre-CQ@@@
************************************************************
** Finished Stage BuildStart - Thu, 15 Sep 2016 15:39:37 -0700 (PDT)
************************************************************
Traceback (most recent call last):
  File "/work/chromiumos/chromite/cbuildbot/builders/generic_builders.py", line 296, in Run
    self.Initialize()
  File "/work/chromiumos/chromite/cbuildbot/builders/generic_builders.py", line 64, in Initialize
    self._RunStage(build_stages.CleanUpStage)
  File "/work/chromiumos/chromite/cbuildbot/builders/generic_builders.py", line 107, in _RunStage
    return stage_instance.Run()
  File "/work/chromiumos/chromite/cbuildbot/stages/generic_stages.py", line 496, in Run
    ready = self.WaitUntilReady()
  File "/work/chromiumos/chromite/cbuildbot/stages/build_stages.py", line 57, in WaitUntilReady
    raise Exception('[xkcd] Die. Die. Why won\'t you die?')
Exception: [xkcd] Die. Die. Why won't you die?

@@@BUILD_STEP@Report@@@
************************************************************
** Start Stage Report - Thu, 15 Sep 2016 15:39:37 -0700 (PDT)
** 
** Summarize all the builds.

[...snip...]

Note that the exception shows up in a stage prior to the actual stage, and that the relevant stage isn't even started.

After the fix, the output for the same failure is as expected:

pprabhu@pprabhu:/work/chromiumos/chromite$ cbuildbot --local -r /cache_ssd/trybot binhost-pre-cq
15:41:47: INFO: cbuildbot was executed with args /work/chromiumos/chromite/bin/cbuildbot --local -r /cache_ssd/trybot binhost-pre-cq

@@@BUILD_STEP@BuildStart@@@
************************************************************
** Start Stage BuildStart - Thu, 15 Sep 2016 15:41:47 -0700 (PDT)
** 
** The first stage to run.
** 
**   This stage writes a few basic metadata values that are known at the start of
**   build, and inserts the build into the database, if appropriate.
************************************************************

@@@STEP_LINK@Builder documentation@http://www.chromium.org/chromium-os/build/builder-overview#TOC-Pre-CQ@@@
************************************************************
** Finished Stage BuildStart - Thu, 15 Sep 2016 15:41:47 -0700 (PDT)
************************************************************

@@@BUILD_STEP@CleanUp@@@
************************************************************
** Start Stage CleanUp - Thu, 15 Sep 2016 15:41:47 -0700 (PDT)
** 
** Stages that cleans up build artifacts from previous runs.
** 
**   This stage cleans up previous KVM state, temporary git commits,
**   clobbers, and wipes tmp inside the chroot.
************************************************************
Traceback (most recent call last):
  File "/work/chromiumos/chromite/cbuildbot/builders/generic_builders.py", line 296, in Run
    self.Initialize()
  File "/work/chromiumos/chromite/cbuildbot/builders/generic_builders.py", line 64, in Initialize
    self._RunStage(build_stages.CleanUpStage)
  File "/work/chromiumos/chromite/cbuildbot/builders/generic_builders.py", line 107, in _RunStage
    return stage_instance.Run()
  File "/work/chromiumos/chromite/cbuildbot/stages/generic_stages.py", line 500, in Run
    ready = self.WaitUntilReady()
  File "/work/chromiumos/chromite/cbuildbot/stages/build_stages.py", line 57, in WaitUntilReady
    raise Exception('[xkcd] Die. Die. Why won\'t you die?')
Exception: [xkcd] Die. Die. Why won't you die?

@@@BUILD_STEP@Report@@@
************************************************************
** Start Stage Report - Thu, 15 Sep 2016 15:41:47 -0700 (PDT)

[...snip...]

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/95d596b17c7ee02a9b65a3dbbcf5ad296da97c60

commit 95d596b17c7ee02a9b65a3dbbcf5ad296da97c60
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Thu Sep 15 22:39:01 2016

cbuildbot: Handle stage failures while skipping / waiting uniformly.

GenericStages.WaitUntilReady is a mechanism for cbuildbot stages to
specify preconditions that the stage waits for, but that should not
strictly be considered part of the stage. GenericStages.HandleSkip is a
mechanism to skip a stage.
This CL ensures that failures in these functions are handled the same
way as failures when actually performing a stage.

BUG= chromium:615467 
BUG= chromium:644436 
BUG= chromium:425249 
TEST=(1) cbuildbot/run_tests
     (2) Inject failure in a stage and verify that exception shows up in
         the logs for that stage.
     (3) Inject failures in HandleSkip for a stage and verify that
         exception shows up in the logs for that stage.

Change-Id: I2aef9a375f3144f8261cbad0a4452e74ce63bfd6
Reviewed-on: https://chromium-review.googlesource.com/386141
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>

[modify] https://crrev.com/95d596b17c7ee02a9b65a3dbbcf5ad296da97c60/cbuildbot/stages/generic_stages.py