Brief summary of the bug. What is it ? Where is it ?

When configuring an alert, the alert form doesn't validate the entered email against the list of Optimize user email addresses

Steps to reproduce:

1. Create an Alert in Cloud
2. Enter a non-Optimize user in the email address

Actual result:

Vague warning message

Expected result:

(OK) valid explanation of why the request was rejected OR
(Better)In-form validation of the email OR
(Best) User picker for selecting an email in cloud modes{}