Show errors when saving /login/settings page

Currently you don't get an error when you save the /login/settings page and it doesn't succeed.

Steps to reproduce:

  1. Create a user, go to /login/settings and set a username for it
  2. Create another user, go to /login/settings and try to set the same username.

Expected result:

error

Actual result:

You don't get an error. It shows the username you tried to save. But when you open the page again with a different (or no) flow ID, you'll see that the username wasn't saved because the field is now empty.

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