add missing failStep()

If a re-run fails today, that leaf exec.Run() step will fail, and its
grandparent (representing a giant bunch of sources and a set of flags)
fails, and the whole great-grandparent task fails, but the exec.Run()'s
parent (a small batch of those sources) stays green.

Here's a before-this-CL example:
https://task-driver.skia.org/td/y2nmMwJAyuK2kmCNGtrk?ifNotFound=https%3A%2F%2Fchromium-swarm.appspot.com%2Ftask%3Fid%3D520ccdac1ff16010

Here's one where the failures propagate up right, I think:
https://task-driver.skia.org/td/Gb2Pta6M2bZAJpQYvRXO?ifNotFound=https%3A%2F%2Fchromium-swarm.appspot.com%2Ftask%3Fid%3D520cfbaeb2f06010

I don't suspect anything's wrong beyond bookkeeping.

Change-Id: Ib138f62f6663bbba1804ccafb28756f8c4c4d3ea
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378401
Reviewed-by: Eric Boren <borenet@google.com>
1 file changed