When a request is made to sync non-existent image, such as one called "fake_image", from a real remote repository such as quay.io - either from the execution environment list view kebab toggle OR from the Sync button on the Remote Registries page - there is no indication that the image is non-existent on that very page.

There is a NOT_FOUND message on the task detail page, but, if one does not navigate there, there could be unnecessary confusion and wasted time if there is a simple typo on the add Execution Environment modal. Better to have a NOT_FOUND message appear right away.

To reproduce:
1. In Remote Registries menu, and a registry with the name "Quay" and url "https://quay.io".
2. In Execution Environments menu, add and EE with the name "fake_image" and upstream "fake_image".
2. From either the EE OR the Remote Registries list view, click "Sync from Registry".
Issue: There is no immediate UI indication (without navigating) that the image you are attempting to sync is non-existent.

How reproducible: always

Expected Behavior
A warning alert appears indicating that the image the user is attempting to sync does not exist in the remote registry. Check spelling or visit the remote registry url to find an image to sync.