Unclear error message when yaml spec is wrong

[mikesol]

It is really easy to write subtly wrong YAML and unmock will fail without any warning.

servers:
- https://example.prismic.io

Should be

servers:
- url: https://example.prismic.io

But unmock does not catch this and as a result, the tests fail later down the line with a cryptic error message.

[idantene]

Do we want to include this as unmock's responsibility, or loas?
I think verification of service specification should be external to unmock. Ideally as a final step in loas, to verify the output is valid OAS.