plumb errors up to mid-level steps

As is, on failure the top-level task fails correctly,
then the next level steps all confusingly look green,
then the next two levels down are correctly failed and red.

I think this is because worker(ctx) fails `ctx`, which we make like
this,

    ctx := startStep(w.Ctx, td.Props(strings.Join(w.Sources, " ")))

but nothing ever fails that `w.Ctx`.  This should fix that.

Compare trybot runs here with https://task-driver.skia.org/td/BMCjZbc5ki1cXbkM0oZp?ifNotFound=https%3A%2F%2Fchromium-swarm.appspot.com%2Ftask%3Fid%3D51aa8032a90a8810

Change-Id: Idfbd933b9027cac423a3a2cc5b0513c894d60e63
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369265
Reviewed-by: Eric Boren <borenet@google.com>
1 file changed