BulkEditUserGroups: Provide helpful technique to prune out erroneous entries

Currently, when entering a large number of users in the right pane ("Add members to selected group(s)") of the "Bulk Edit Group Members" page, if there's a problem with any user, the operation fails with no action taken

This is a huge pain when trying to keep a large list in sync. For example, if I have a huge group (say 300 members), and a list of 40 that I want to make sure are added, and only 15 actually need to be added, I have to manually prune the other 25 out of the list.

The most helpful thing to do in this case is to still return the error with no action taken. However, instead of returning the exact same list as entered, prune the erroneous entries out of the list (or at least, provide /some/ way of getting that list)