Regression: Errors while adding a new channel are no longer shown in the UI

When an error occurs when adding a new channel, an i18n'd error message used to be shown in the UI. Such messages are no longer shown in the UI. Errors are only logged, but the user does not get any feedback.

Steps to reproduce:
1. start the archetype
2. log in to the CMS as admin
3. navigate to the channel manager
4. click the button 'Add Channel'
5. select the blueprint 'Subsite' and click 'Next...'
6. enter a random name
7. enter the URL "http://localhost/foo/bar"
8. click 'Create Channel'

Expected behavior: an error message is shown saying "The URL ' http://localhost/foo/bar ' cannot be used because the parent URL ' http://localhost/foo ' is not used by any channel." (see the screenshot error-creating-channel.png)

Actual behavior: the Channel Properties dialog is closed without any feedback to the user.