Error message is incorrect when you cannot add permissions because you lack permissions yourself

[nellh]

Describe the bug When adding permissions to a dataset, if you lack the edit permissions access, you will instead get an error "User with that email address not found" which is misleading.

To Reproduce Steps to reproduce the behavior:

1. Login as a user with read or edit access to a dataset
2. Try to share the dataset with an existing OpenNeuro user

Expected behavior This should identify the lack of permissions to share access.