Thumbnail Error Responses are 200 OK and `text/plain`

[Nerixyz]

Animated webp links aren't resolved, so requesting the thumbnail for such link will result in an error. However, the HTTP status code of the response is 200 OK (even though the response says 404). Furthermore, the content-type is text/plain but it's clear that JSON is returned.

Example link:
https://braize.pajlada.com/chatterino/thumbnail/https%3A%2F%2Fcdn.7tv.app%2Femote%2F60afbfbaa3648f409a6e5211%2F4x

[pajlada]

I can confirm the issue that you've noted.

I'm working on fixing this, however, this required a pretty big overhaul as the values stored in the cache needs to include the status code and content type now.

As a result of this, Chatterino2 should be updated as well to handle non-200 codes better (i.e. handle onError in src/providers/LinkResolver.cpp) to output the message string if it exists in the json response, if the content type was application/json

[pajlada]

Thumbnails should return error codes (e.g. 500 or 404) if we can't produce a valid thumbnail

Normal tooltips we create should, in most cases, return 200 even if the underlying YouTube video did not exist.
The tooltip was created correctly. status informs the API user what sort of data to expect.

(just explaining for myself xd)