Cannot choose "no such message" when searching for images

[MrOrz]

As found in 20220921 meeting, the following button does not work:

Root cause

In POSTBACK_NO_ARTICLE_FOUND logic of choosingArticle handler, we currently do not handle images properly.

Proposed solution

Should do both of the following:

• Copy the current not-found reply ("I am sorry you cannot find...") from processImage to choosingArticle
  • separate image handling logic from text handling logic within choosingArticle
• Update the wording of not-found reply in processImage to something like "Unfortunately, I currently don’t recognize..." in initState.
  • The current wording in processImage is actually the 'not found' reply for text when choosing article.