Remove deleted user from Workgroup

[ARiis-63]

When trying to remove a user from a workgroup membership, who has already been deleted from system.
We do get this error:
user ID is NOT a known ERDA user and Workgroup participation is limited to users already signed up to ERDA.

[jonasbardino]

Thanks for registering the issue @ARiis-63 .

For the record, it should be possible to implement with some tweaking of the shared init_vgrid_script_add_remove function in mig.shared.vgrid to toggle hard error on missing ID specifically when called from the remove owner/member backends. The default is already like for the similar remove resource case.