Improve error message on malformed step

[kichristensen]

What does this change

Instead of giving a cryptic error message, "more than one mixin specified", better describe the most likely cause of the problem, making it easier for people to understand what to fix.

What issue does it fix

Closes #370

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR

[schristoff]

Nit: This package seems to be not very maintained, but it doesn't seem to really need a lot. We've been using it already, so it's really nbd, but with recent security things going on I think I'm just hyper-aware for a bit :)
Just thinking out loud here.