Handling of errors when adding items in Bulk

[page200]

When adding new items via "Bulk" (as opposed to "Singular"), the user should be notified about items that couldn't be added. For example, those lines of text can remain in the window (with a similar message to when an error happens in the "Singular" tab), whereas successfully added ones can disappear.

Not notifying the user is particularly unexpected when they try to add something like
https://www.youtube.com/c/oceanexplorergov/videos
which the user might expect to work but which doesn't work currently as opposed to
https://www.youtube.com/user/oceanexplorergov