Externally loaded schemas do not report a failure to load.

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Q&A (please complete the following information)

• OS: Windows
• Browser: Firefox and Chrome
• Version: 64.0 and 71.0 respectively
• Method of installation: npm
• Swagger-UI version: swagger-ui-dist@3.20.4
• Swagger/OpenAPI version: OpenAPI 3.0.0

Content & configuration

Example Swagger/OpenAPI definition:

openapi: "3.0.0"

info:
  description: "Something"
  version: "1.2.0"
  title: "Title"
  termsOfService: ""
  contact:
    email: "dude@something.com"
  license:
    name: "Proprietary"

servers:
  - url: 'https://dev-superduperserver.com'
    description: Development server

security:
  - basicAuth: []

paths:
  /apath:
    get:
      operationId: doAThing
      responses:
        '200':
          content:
            application/json:
              schema:
                oneOf:
                  - "$ref": "/jsonschemas/somethingThatIsBlank.json"
                  - "$ref": "/jsonschemas/somethingThatGives404.json"

Swagger-UI configuration options:

import {SwaggerUIBundle, SwaggerUIStandalonePreset} from 'swagger-ui-dist'
import './node_modules/swagger-ui-dist/swagger-ui.css'

const ui = SwaggerUIBundle({
	url: "/openapi.yaml",
	dom_id: '#swagger-ui',
	deepLinking: true,
	presets: [
		SwaggerUIBundle.presets.apis
		,SwaggerUIStandalonePreset
	],
	plugins: [
		SwaggerUIBundle.plugins.DownloadUrl
	],
	layout: "StandaloneLayout"
	,validatorUrl: null
})

?yourQueryStringConfig

Describe the bug you're encountering

The interface does not display errors correctly when the server sends an blank response body schema or a 404 for an external schema.

I should note, CORS errors are reported correctly, which is nice.

To reproduce...

Steps to reproduce the behavior:

1. Have the server return a 200 with an empty response body to have the spinners spin forever.
2. Have the server return a 404 with an empty response body to have an empty schema appear with no errors.

Expected behavior

A 200 with no response body should yield some sort of warning, because the schema should at least be {}
A 404 from the server should yield a big red error somewhere.

Screenshots

When server returns 200 and empty body:

When server returns 404:

Additional context or thoughts

This is a nice bug report template, good work.