[Audio block] Unsupported url results in empty audio file UI

[jd-alexander]

Describe the bug
When you add an unsupported url to the Audio Block via the Insert from URL media upload action, the block does not present a warning, it shows an empty file.

To Reproduce
Steps to reproduce the behavior:

1. Add an Audio block.
2. Select Insert from URL.
3. Insert an URL like http://www.mp3.com/music/
4. Notice that an empty audio file is shown.

Expected behavior
The Audio block should mark this URL as invalid and show a warning.

Screenshots

RPReplay_Final1621445109.MP4

screen recording from @fluiddot