Misleading error message when response description is missing

[bburtin]

Describe the bug
When I omit the description field from a response, the error I get is "200 property should have required property $ref". It would be better if spectral told the user that the description field is missing.

To Reproduce

Remove the description field from a response and run spectral lint

$ npx spectral lint test.yaml
OpenAPI 3.x detected

.../test.yaml
  1:1   warning  oas3-api-servers       OpenAPI `servers` must be present and non-empty array.
  1:1   warning  openapi-tags           OpenAPI object should have non-empty `tags` array.
  2:6   warning  info-contact           Info object should contain `contact` object.
  2:6   warning  info-description       OpenAPI object info `description` must be present and non-empty string.
  8:9   warning  operation-description  Operation `description` must be present and non-empty string.
  12:9  warning  operation-tag-defined  Operation tags should be defined in global tags.
 14:15    error  oas3-schema            `200` property should have required property `$ref`

Expected behavior
spectral should tell me that the description field is missing

Environment (remove any that are not applicable):

• @stoplight/[email protected]
• OS: Ubuntu 18.04

Additional context

$ cat test.yaml   
openapi: "3.0.0"
info:
  version: 1.0.0
  title: REST API

paths:
  /test:
    get:
      summary: List
      operationId: list
      tags:
      - Greenhouse
      responses:
        '200':
          content:
            application/json:
              schema:
                type: object

[mloberg]

This is probably related to #403

[philsturgeon]

Yep sorry about this. It's a duplicate of #403 so let's chat over there.