Email testing endpoint should return HTTP error code on error (was: make the emails configuration UI better)

From @nicofrand suggestions in !450 (merged):

• Maybe the request to /api/v1/settings/test-email/ should return a different HTTP status (an error code) ?

• 3 buttons are a bit confusing I think. What about having a "clear" button right next to the input, and only "Send a test email" and "Save" buttons on the bottom ? Ie., to remove the email, you clear the field and save.

• There should be a placeholder in the email field.

• There should be a message that says "A test email was sent" once the button is clicked.