Add an informative stub in case the AsyncAPI document is invalid

[KrySeyt]

Description

Hi! I use this component via CDN to generate a schema from json/yaml. Currently, if the AsyncAPI document is invalid, just a blank page is displayed. It would be nice if this component displayed some error information, as in AsyncAPI Studio

CDN version - https://unpkg.com/@asyncapi/[email protected]/browser/standalone/index.js

Reasons

It will be easier to debug the case when nothing is displayed.

[github-actions]

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