Missing media edit form validation callback

Quick solution, but I think it must be moved to some additional validation method

I've found the better variant and added the modal with success messages

Thank you for your contribution.

Sadly, I just saw this issue after the beta4 release :(

Looks good and in-line with AddMediaFormBase. Also my tests were fine.

Ah - and sorry for missing your username in the commit msg :(

Too many non-smiling similes here

Automatically closed - issue fixed for 2 weeks with no activity.