Make it clear when a deletion is cancelled due to unset checkbox
Currently media deletion happens like this:
1. Go to media page.
2. Click "delete".
3. Check the "I am sure" checkbox.
4. Click "delete permanently".
If the checkbox at step 3 is not checked, you are back at step 1
(the media page appears untouched).
MediaWiki and file managers do it differently: you don't have to
use a checkbox, but only click another button.
When the checkbox UI is used, but the deletion is cancelled due to
the checkbox being unset, a message should appear to the user, or
he might think that the deletion simply failed.
P.S.: I suppose that checkbox is there because people used to
deleting (or running a bot) may click through many buttons and
accidentally delete stuff. However, when everything uses a
checkbox, it will just slow deletion down and make the user get
used to that longer path. There should probably be a way to delete
larger arbitrary groups of media quicker, at least where it is
possible to upload large amounts of media.