ArcGIS Online user password reset - notify if invalid username

[KatieMilne]

The current reset password interface for ArcGIS Online does not notify the user if their username does not exist. This has caused frustration for our users who think an email is coming but it never arrives as they had a typo in their username.

It would be helpful to notify the user if the user name is invalid, and perhaps to show the email domain that the email is sent to or the organisation that the user belongs to.