Opening a diagram that doesn't exist should display a proper error

[CamilleLetavernier]

Original issue: eclipsesource/ecore-glsp#124

Currently, when opening a diagram that doesn't exist (e.g. because the file was deleted), the Ecore editor is still "properly" opened in Theia, but is unusable (all actions are disabled). It seems that in this case, there are some unhandled exceptions on the backend, causing random issues.

This can easily be reproduced if an Editor was open in Theia, then deleted from the file system. When reopening Theia, it will attempt to load the editor again, even though the file no longer exists.