[FEATURE] Failing story validation does not tell which property is unevaluated

[dblock]

Is your feature request related to a problem?

After #362 if a story has an unevaluated property the error doesn't say which property is unevaluated.

In the following example synopsis is invalid.

prologues:
  - synopsis: Create an index named `books` with mappings and settings.
    path: /{index}
    method: PUT
    parameters:
      index: books
    request_body:
      payload: {}

When running tests.

ERROR   cat/indices.yaml (Invalid Story: data/prologues/0 must NOT have unevaluated properties)

What solution would you like?

Similar to the use of ajv_errors in the response body/schema validator.

[nhtruong]

I'm not a fan of having to inject error message instructions into every schema to get the proper message with ajv_errors. It's too cumbersome and relies on our schema traversing mechanism not missing anything. So far, it's easy to pinpoint where the violation happens with ajv.errorsText except for additionalProperties and unevaludatedProperrties errors. A solution I'm thinking of:

const validate = ajv.compile(schema)
if(!validate(data)) {
 const { prohibited_props,  others } = validate.errors.reduce((result, error) => {
    if (is_prohibited_prop_error(error)) result.prohibited_props.push(item);
    else result.fail.push(item);
    return result;
  }, { prohibited_props: [], others: [] });
  
  return ajv.errorsText(others) + ' ' + prohibited_props.map((e) => prohibited_prop_message(e)).join(' ')
}

[dblock]

I think any solution is fine. We probably do want to consolidate code that invokes ajv into one class for reuse.

[nhtruong]

I'll handle this