When making structural mistakes on changing a step's presentation, strange and hard-to-debug errors are shown for the subsequent step.

Example test case:
https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1016284

See failure output:
https://logs.chromium.org/v/?s=infra%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8948913736474745088%2F%2B%2Fsteps%2Fpresubmit%2F0%2Fstdout

It would be nice if the step.presentation could catch errors on change.