Fix an email address validation UI feedback issue when creating new users

Summary: On the "New User" web workflow, if you use an invalid email address, you get a failure with an empty message.

Test Plan:

• Before: Tried to create a new user with address "asdf". Got no specific guidance.
• After: Got specific guidance about email address formatting and length.

Differential Revision: https://secure.phabricator.com/D21264