Non-existing widget: Better error handling

[vestimir]

We need a better handling when a widget is present in the JSON structure, but there's no corresponding React file.

Steps to reproduce

1. Create dummy widget with igor
2. Set it up on a page and save
3. Delete the dummy widget
4. The CMS will crash with undefined

We need to catch when a widget cannot be found and to display relevant message in the browser console.

This issue affects:

• canvas
• reorder dialog