Media Grid: display upload errors in a dismissable sidebar.

See #29037.

Built from https://develop.svn.wordpress.org/trunk@29415


git-svn-id: http://core.svn.wordpress.org/trunk@29193 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid: after [28996], use the same cover size for audio/video files that images use for thumbnails.

See #24716.

Built from https://develop.svn.wordpress.org/trunk@29416


git-svn-id: http://core.svn.wordpress.org/trunk@29194 1a063a9b-81f0-0310-95a4-ce76da25c4cd