Improve user creation failure screen when mail already exists

When creating 2 users with different logins but same mail address and one does not specify passwords, a HTTP 500 is generated, but no reaction in UI

Request that a better error message happens.

To reprouduce:

• install nuxeo 9.10 or 10.10 and (fake) SMTP feature OK and login as Administrator
• create user test1 mail test@test.com and do not define password => OK => accept the invitation from a mailer
• create user test2 mail test@test.com and do not define password => nothing happens
  Underneath, in network browser console or in server.log, you can see HTTP 500and exception being thrown, but UI remains indifferent.

Notice: nightly does not seem to enable user creation