When email fails to existing domain managers on new domain manager add, should display warning message, and not block

[dave-kennedy-ecs]

Current Behavior

When adding a domain manager, system attempts to send emails to all existing domain managers before updating registrar.
If any of the emails to existing domain managers fails, the entire action fails midstream; registrar is not updated, and an unknown number of emails may have already been sent.

Expected Behavior

When adding a domain manager, system attempts to send emails to all existing domain managers before updating registrar.
If any of the emails to existing domain managers fails, the action continues. And at the end of the action, when a user is presented with messages, the user is presented with a warning message that says "Could not send email confirmation to existing domain managers." It doesn't need to list the emails of who it failed for, but that should be included in an error log.

Steps to Reproduce

1. Create user with an email address that is not on the white list.
2. Add that user as a domain manager on a domain
3. Attempt to add a different email address (which is on the white list) as a new domain manager

Environment

No response

Additional Context

No response

Issue Links

Blocked by: #2370