Support open closed projects with missing .project file

Currently when the .project file was deleted, the project is shown as closed in Eclipse (what is somehow fine).

But when one then tries to open the project a nasty error dialog is shown:

> The project description file (.project) for ''{0}'' is missing. This file contains important information about the project. The project will not function properly until this file is restored.

but we already store the important data in the workspace area with https://github.com/eclipse-platform/eclipse.platform/pull/2033

a much better approach therefore would be to restore as much data as we can and show a warning and we actually have already

> The project description file (.project) for ''{0}'' was missing. This file contains important information about the project. A new project description file has been created, but some information about the project may have been lost.