No error for invalid usernames on bulk add to usergroup

Adding users to a group will fail silently for invalid usernames that are supplied alongside valid usernames.

This problem is exhibited in:

• ACP -> Users and Groups -> Manage groups -> Members
• UCP -> Usergroups -> Manage groups -> Manage users

For example, when adding users to a group if you enter:

valid1
valid2
invalid3

And then submit, where valid1 and valid2 are users that exists and invalid3 is a user that doesn't exist, the user making the additions will see a "success" message in the ACP or UCP.

This is misleading, because the user will assume that the operation was successful and will likely click away to a different screen - so in the event of a typo on one or more of the usernames, not all of the users they intended to add to the group will have been added.

A failure message containing the list of invalid usernames is a more suitable call-to-action, as the user will be made aware of the problem they need to correct.