Media blocks: Show error message for invalid URLs

[thehenrybyrd]

Describe the bug
When adding an invalid URL in an Audio block (via "Insert from URL"), it'd be nice to show on error message to let the user know they haven't typed a valid URL, before they tap "apply"
Currently, it shows an error after tapping "apply".
This means if someone typed the URL slightly wrong, they have to go back in an re-type the whole thing.

To Reproduce
Steps to reproduce the behavior:

1. Use a WordPress.com site with a plan
2. In a Post or Page, add an Audio block
3. Tap "Insert from URL" and type an invalid URL or just some random characters

Expected behavior
I would expect the URL to be validated while I'm typing and alert me if it's invalid before I can tap "apply".

Screenshots

Smartphone:
Samsung Galaxy S21, Android 11, WPAndroid 17.5-rc-1

Additional context
Found in beta testing: p5T066-2lr#comment-8584

[SiobhyB]

I tweaked the title here to reflect that this is an enhancement that would benefit all media-based blocks, including VideoPress. It was brought up here that requiring users to go back through the process of adding a URL is a pain point: p5T066-3Xm-p2#comment-14600