Address `User has already been taken` errors for invocations of `grant_revoke_gem_authority.rb`

[jrgriffiniii]

When invoking the script bundle exec ruby ./script/grant_revoke_gem_authority.rb, I have encountered errors of the following form:

active-fedora -a $EMAIL_ADDRESS_1 -a $EMAIL_ADDRESS_2 -a $EMAIL_ADDRESS_3 [...]: Adding $EMAIL_ADDRESS_1: User has already been taken

I am not quite certain what is raising these errors, but the errors seem to fail to adjust user permissions for Gems for a single given user when a list of users are given (i. e. while there may be issues for revoking the authority from $EMAIL_ADDRESS_1 for active-fedora, this should then continue to attempt to revoke authority for $EMAIL_ADDRESS_2 from active-fedora).

[jrgriffiniii]

Resolved with #123