Refine error message when embedding invalid Video (URL)

[Presskopp]

like in #40771 you get the following error when your link is invalid:

Sorry, we can’t display the video file type selected. Please select a supported video file (...) or stream (YouTube or Vimeo) instead.

When posting an incomplete youtube link for example it's not the file type but a character wrong or missing.

I propose to change the error message like in the patch attached:

Sorry, either the selected file type is not supported or the video stream is invalid. Please make sure to select one of the following video file types (%1$s) or a valid stream (YouTube or Vimeo) instead.

[jnylen0]

This is good as an incremental improvement, and worth committing by itself IMO, but I think we can probably go a bit further here. What are the failure conditions we expect? For example:

• Non-video file type (can we show the actual file type we received instead?)
• Non-200 response (can we show the response code or some text about it instead?)
• Response failed, blocked by browser (#40771; can we detect this?)