Correctly render errors when adding account

[claucece]

There are three errors that can show up when adding an account:

1. Cannot add account if account field is missing.
2. XMPP address has an invalid domain part. An XMPP address should look like this: local@domain.com.
3. XMPP address has an invalid local part. An XMPP address should look like this: local@domain.com.

Currently, error 1 with any of the other errors will stack rather than replace the notification.

Error 2 and error 3 will not replace themselves nor stack. For example, if an user forgets to add the local part, error 3 will show. If he then forgets to add the domain part, error 3 will still be there and will not be replaced by error 2.

Expected behavior: errors should replace themselves or there should be only one error, such as "invalid credentials".