Domain add domain manager, when existing domain manager, sends email before erroring

[dave-kennedy-ecs]

Current Behavior

In Domain management, Domain Managers tab, Add a domain manager
Attempt to add a domain manager, for an existing user who is already a domain manager for the domain
An email is sent to the domain manager, and then an error is presented to the user

Expected Behavior

When the existing user is added, no email should be sent, and the AlreadyDomainManagerError should be raised by the code in send_domain_invitation_email

Steps to Reproduce

1.Manage a Domain, and navigate to Domain managers
2.Add an email address that is already listed as a domain manager

Environment

No response

Additional Context

Note in the code snippet above that the checks for existing invitations only checks for retrieved domain invitations, and doesn't account for users who were directly given the UserDomainRole without an invitation

Issue Links

No response