The last step in a build on the waterfall, for instance here:

https://uberchromegw.corp.google.com/i/chromeos/builders/lulu-release/builds/1800

is titled "failure reasons" and has two links, "stdio" and "reason".  I haven't checked yet how common this is, but at least for this build, the content of those URLs is not useful.  The first contains this:

====

@@@STEP_CURSOR@Failure reason@@@
This build is configured to send log data exclusively to LogDog. Please use the LogDog link on the build page to view this log stream.
@@@STEP_STARTED@@@
@@@STEP_FAILURE@@@
@@@STEP_LINK@logdog-->reason@https://logs.chromium.org/v/?s=chromeos%2Fbb%2Fchromeos%2Flulu-release%2F1800%2F%2B%2Frecipes%2Fsteps%2FFailure_reason%2F0%2Flogs%2Freason%2F0@@@
LogDog Link [reason]: https://logs.chromium.org/v/?s=chromeos%2Fbb%2Fchromeos%2Flulu-release%2F1800%2F%2B%2Frecipes%2Fsteps%2FFailure_reason%2F0%2Flogs%2Freason%2F0
@@@STEP_LOG_END@reason@@@


--------------------------------------------------------------------------------
started: Tue Dec 26 23:32:16 2017
ended: Tue Dec 26 23:32:16 2017
duration: 0 secs
status: FAILURE
status reason: 

====

the other has this single line:

Step('cbuildbot [lulu-release]') failed with return_code 1